The Application of Polypropylene Membranes for Membrane Distillation-Juniper Publishers
Authored
by Marek Gryta
Abstract
Microporous hydrophobic membranes are applied in
membrane distillation. During a long-term of module exploitation, a part
of the pores is filled by water, and the membrane wetting causes that
the industrial implementation of this process is hindered. Several
investigators achieved promising results using the polypropylene
membranes formed via TIPS method for various applications of membrane
distillation.
Keywords:Membrane distillation; Polypropylene membrane; Hydrophobic membrane Abbrevations: MD: Membrane Distillation; PTFE: Polytetrafluoroethylene; PP: Polypropylene PE: Polyethylene; TIPS: Thermally Induced Phase Separation
Introduction
Membrane distillation (MD) is an evaporation process
of water through non-wetted porous membranes. In this process the salts
and other non-volatile compounds present in the feed water are retained
and the quality of produced distillate is close to distilled water
[1-4]. The results of studies presented in the literature indicate that
the MD process is not only an effective method for water desalination
[1-3, 5], but also can be applied for wastewater treatment, especially
when the salts concentration is high [3,6,7].
A huge amount of energy is required for water
evaporation; thus, a good thermal efficiency is important for industrial
implementation of the MD process [7,8]. The capillary MD modules
allowed to obtain the thermal efficiency at a level of 70-80% [9].
Moreover, as MD can be operated with low-grade heat, the coupling of MD
with waste energy makes the MD process very attractive [8,10].
The porous hydrophobic membranes are assembled in the
membrane modules and the membrane wetting is the major problem of MD
process [1-3]. The membrane wettability may be accelerated by scaling
and fouling [2,3]. The possibility of NaCl solutions concentration in
the MD process up to the saturation state has been already demonstrated
many times for different types of the hydrophobic membranes [3,10,11].
However, during the separation of actual brines, which contain besides
NaCl also
hardly soluble salts, a serious problem is scaling (precipitation e.g.
CaCO3 and CaSO4) [2,12,13]. Therefore, the high
salt concentrations in the feed water (scaling) may restrict e.g. the
fresh water production from brines [2,3,12]. For this reason, the number
of works presenting the preparation of MD membranes with enhanced
resistance to wetting has significantly grown [7,11,14-16]. However, the
performance and durability of these membranes is most often tested over
a period of below 10-50h, hence, the MD process has been used so far on
the pilot scale for desalination with utilization of the traditional
membranes made of polytetrafluoroethylene (PTFE), polypropylene (PP) and
polyethylene (PE) [1,5,9,17].
A promising method to mitigate fouling and scaling
intensity is the application of low feed temperature [12], however, a
large membrane area should be used in order to achieve a high efficiency
of the installation [18]. Thus, the realization of industrial
implementation requires the membranes as cheap as possible, namely,
manufactured by a simple method from inexpensive raw materials [11,19].
Such conditions are fulfilled by the capillary membranes produced from
polypropylene by a TIPS method [3,20]. The TIPS process parameters and
the diluents type and its concentration in the initial polymer/diluents
system affected considerably the phase separation behaviors and the
final membrane microstructure for PP membranes [21,22]. The PP membranes
manufactured in the industrial installation were presented in Figure 1.
The small increase (from
30 to 35wt%) of PP concentration in the casting solution caused
significant changes in the pores structures. This showed while
such a different membrane morphology is obtaining during the
PP membranes preparation via TIPS method [20-22]. For this
reason, only a few kinds of membranes produced from PP are
appropriate for the MD process (e.g. Accurel PP [2,9,16,23]).
The promising results were achieved using PP membranes
for various applications of MD process [2,9,24]. A disadvantage
of the PP membranes is a formation of the hydrophilic groups
on their surface during MD [22]; as a result, the membrane
surface was wetted after 40-50h of MD process operation [2].
However, in spite of a rapid wetting of the surface, the pores
were not wetted over the entire membrane cross-section during
long-term MD studies, which confirmed that the capillary PP
membranes exhibit the excellent resistance to wetting over a
period of 2-4 years of MD module exploitation [9,25].
Conclusion
The polypropylene membranes formed via TIPS method can
be applied for MD process. However, the membrane morphology
is strongly affected by the TIPS conditions. Therefore, not
all hydrophobic PP membranes produced for microfiltration
process are appropriate for MD. The good results were obtained
applying the Accrual PP membranes for MD process.
In the pilot scale studies of seawater desalination, an
intensive scaling caused the need to replace the membrane
modules after several months of operation [21]. Although the
scaling can be limited by lowering the feed temperature below
50 ͦC, but the operational efficiency will be several times lower
[12]. In this case it is necessary to increase the membrane area
in the installation, hence, the application of cheap membranes
such as the proposed capillary PP membranes allows to reduce
the production costs.
For more
details Journal of Polymer Science please
click on: https://juniperpublishers.com/ajop/index.php
To read more…Full Text in Juniper Publishers click on https://juniperpublishers.com/ajop/AJOP.MS.ID.555565.php
Comments
Post a Comment