Nano-Porous Graphene as Free-Standing Membranes
47
sheet membrane, with a spacer between each support and membrane to drive the
feedthrough each membrane, while flowing the permeate outside the module [71–
75]. The advantage of this type of module is fouling resistivity, but it has a low packing
density which demands a lot of space and makes it expensive. Tubular membrane
units, another RO configuration, comprise membrane tubes wrapped around punctured stainless-steel cylinders. The feed runs across the tubes and the permeate
flows out from the membrane and support [76]. These two modules share the same
advantages and disadvantages and are mainly used for extreme foul feeds [77].
In contrast, hollow-fiber modules have a large packing density, where multiple
fine hollow-fiber membranes are placed in a pressure vessel, and the feed runs outside
the membranes [72, 73]. While these units have high productivity, they are highly
susceptible to fouling. Hollow fiber modules are characteristically 10–20 cm in diameter and 1.0–1.6 m long [78]. One of the most extensively used modules nowadays, is
the spiral-wound. A spiral-wound module contains plane layers of membrane separated from each other by spacers and are wrapped around a punctured collection
tube. The feed solution passes across the membrane surface and a portion of the feed
permeates into the membrane envelope where it spirals toward the center and exits
through the collection tube as shown in Fig. 1 [78]. The spiral-wound module has
a high packing density in addition to reasonable fouling resistance. Costly wise, it
is the cheapest module setup produced from TFC membranes [79, 80]. Advances
in materials, manufacturing techniques, feed channels and the size of vessels and
spacers in recent years have contributed to better adjustment of the internal fittings
between module elements and liquid transport features in this module, thus the multi
envelope design of spiral-wound module minimizes the pressure drop encountered
by the permeate traveling toward the central pipe and also minimizes fouling [78, 81].
The standard industrial spiral wound module is 20.3 cm in diameter and 101.6 cm
long [78]. Four to six spiral wound membrane modules are normally connected in
series inside a single pressure vessel. A typical 20.3 cm diameter tube containing six
modules has 100–200 m
2 of membrane area [78]. Spiral-wound is superior to the
other three modules in terms of lowest energy use, lowest system cost and highest
design flexibility but stands in third place in terms of susceptibility to fouling after
plate and frame and tubular modules [81].
2.3 Ideal Membranes, Properties, and Prospects
The rise of graphene and other 2D materials in the past 15 years [82], has opened
new avenues in membrane technology. The atomic thickness of these materials makes
them the thinnest possible barrier, and if combined with their remarkable mechanical strength, chemical robustness, extreme flatness, and ability to sustain selective,
nanometre-scale pores [21, 27, 83, 84] at pristine form, become ideal candidates for
replacing the current state-of-the-art membranes. Key advantages of nano-porous
atomically thin membranes as active layers in RO can be summarized as follows
[20]:
47
sheet membrane, with a spacer between each support and membrane to drive the
feedthrough each membrane, while flowing the permeate outside the module [71–
75]. The advantage of this type of module is fouling resistivity, but it has a low packing
density which demands a lot of space and makes it expensive. Tubular membrane
units, another RO configuration, comprise membrane tubes wrapped around punctured stainless-steel cylinders. The feed runs across the tubes and the permeate
flows out from the membrane and support [76]. These two modules share the same
advantages and disadvantages and are mainly used for extreme foul feeds [77].
In contrast, hollow-fiber modules have a large packing density, where multiple
fine hollow-fiber membranes are placed in a pressure vessel, and the feed runs outside
the membranes [72, 73]. While these units have high productivity, they are highly
susceptible to fouling. Hollow fiber modules are characteristically 10–20 cm in diameter and 1.0–1.6 m long [78]. One of the most extensively used modules nowadays, is
the spiral-wound. A spiral-wound module contains plane layers of membrane separated from each other by spacers and are wrapped around a punctured collection
tube. The feed solution passes across the membrane surface and a portion of the feed
permeates into the membrane envelope where it spirals toward the center and exits
through the collection tube as shown in Fig. 1 [78]. The spiral-wound module has
a high packing density in addition to reasonable fouling resistance. Costly wise, it
is the cheapest module setup produced from TFC membranes [79, 80]. Advances
in materials, manufacturing techniques, feed channels and the size of vessels and
spacers in recent years have contributed to better adjustment of the internal fittings
between module elements and liquid transport features in this module, thus the multi
envelope design of spiral-wound module minimizes the pressure drop encountered
by the permeate traveling toward the central pipe and also minimizes fouling [78, 81].
The standard industrial spiral wound module is 20.3 cm in diameter and 101.6 cm
long [78]. Four to six spiral wound membrane modules are normally connected in
series inside a single pressure vessel. A typical 20.3 cm diameter tube containing six
modules has 100–200 m
2 of membrane area [78]. Spiral-wound is superior to the
other three modules in terms of lowest energy use, lowest system cost and highest
design flexibility but stands in third place in terms of susceptibility to fouling after
plate and frame and tubular modules [81].
2.3 Ideal Membranes, Properties, and Prospects
The rise of graphene and other 2D materials in the past 15 years [82], has opened
new avenues in membrane technology. The atomic thickness of these materials makes
them the thinnest possible barrier, and if combined with their remarkable mechanical strength, chemical robustness, extreme flatness, and ability to sustain selective,
nanometre-scale pores [21, 27, 83, 84] at pristine form, become ideal candidates for
replacing the current state-of-the-art membranes. Key advantages of nano-porous
atomically thin membranes as active layers in RO can be summarized as follows
[20]: