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Membrane Processes
A membrane is a selective barrier that permits the
separation of certain species in a fluid by
combination of sieving and diffusion mechanisms
Membranes can separate particles and molecules
and over a wide particle size range and molecular
weights
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Membrane Processes
Four common types of membranes:
Reverse Osmosis
anofiltration
!ltrafiltrationMicrofiltration
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"he R#O# membrane is semi$permeable with
thin layer of annealed material supported on amore porous sub$structure# "he thin s%in is
about '( micron thic% and has pore size in the
( ) *& Angstrom range# "he porous sub$structure is primarily to support the thin s%in#
"he pore size of the s%in limits transport to
certain size molecules# +issolved ions such asa and ,l are about the same size as water
molecules#
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-owever. the charged ions seem to be repelled by
the active portion of the membrane and water isattracted to it# /o adsorbed water will bloc% the
passage and e0clude ions# !nder pressure
attached water will be transferred through the
pores#
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anofiltration is a complementary process to
reverse osmosis. where divalent cations and
anions are preferentially re1ected over the
monovalent cations and anions# /ome organics
with M2 3 *&& $(&& are removed "here is an
osmotic pressure developed but it is less than that
of the R#O# process#
Microfiltration and !ltrafiltration are essentially
membrane processes that rely on pure strainingthrough porosity in the membranes# Pressure
re4uired is lower than R#O# and due entirely to
frictional headloss
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-ollow fiber:
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/piral wound
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,eramic Membrane 5lements
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/piral !F system
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Pressure re4uirements are based on osmotic pressure
for R#O#. osmotic pressure and fluid mechanicalfrictional headloss 6straining7 for nanofiltration. and
purely fluid mechanical frictional headloss
6straining7 for ultra$ and microfiltration#
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8f clean water and water with some concentration
of solute are separated by a semi$permeable
membrane 6permeable to only water7 water will be
transported across the membrane until increases
hydrostatic pressure on the solute side will force
the process to stop#
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"he osmotic pressure head 6at e4uilibrium7 can be
calculated from thermodynamics#
"he chemical potential 69ibbs free energy per
mole7 of the solvent and the solute6s7 in any phase
can be described as:
&i i i6".p7 R"ln: = +
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2here is the ;standard
state< free energy of a pure solvent or
solute at " and p 6usually '(&, and *
atm7# i= mole fraction of solvent or
solute#At e4uilibrium for the solute and pure
solvent system. respectively:
&6".p .:7 6".p7 + =
i&6".p7
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& &6".p 7 R"ln: 6".p7 + + =
&>
p& &
p
6".p 7 6".p7 dp+
+ =
d9 /d" >dp= +
Because:
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&
p
p > dp R"ln: &
+
+ =
&> R"ln: &+ =
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'total
nR "> R " ,==
After some algebraic manipulation:
osmotic pressure=
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2ater flu0 through the membrane is the most
important design and operational parameter# e0tmost important is solute e0clusion# /ome solute will
diffuse 6by molecular diffusion7 through the
membrane because there will be a significant gradientof the solute across the membrane#
wF ?6 p 7=
Water Flux:
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/olute transport is complicated by the type of ions
being transported# "ransport is generally modeled
by :
s * 'F @ 6, , 7=
Fs= salt flu0 6gcm')sec7
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Applications of Micro$ and !ltrafiltration:
,onventional water treatment 6replace all processes
e0cept disinfection7#
Pretreat water for R#O and nanofiltration#
8ronManganese removal 6after o0idation7#
Removal of +@P precursors#
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Applications for R#O# and nanofiltration:
R#O# application mostly desalination#
anofiltration first developed to remove hardness#
anofiltration can be used to remove +@P
precursors
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Operating pressure ranges:
R#O#F: B& ) C&& psig
MF!F: ( ) C& psig
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Fouling of membranes due to accumulation ofsoluteparticulates at the membrane interface has to be
addressed for economic reasons# "he membranes are
too e0pensive to be replaced for reasons of fouling#
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Fouling issue
"raditional membrane technology is generally
affected by fouling# "his long$term loss inthroughput capacity is due primarily to the formation
of a boundary layer that builds up naturally on the
membrane surface during the filtration process# 8naddition to cutting down on the flu0 performance of
the membrane. this boundary or gel layer acts as a
secondary membrane reducing the native designselectivity of the membrane in use# "his inability to
handle the buildup of solids has also limited the use
of membranes to low$solids feed streams#
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Fouling
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"here are various ways to reduce this fouling such as:
Periodic pulsing of feed
Periodic pulsing filtrate 6bac%washing7
8ncreasing shear at by rotating membrane>ibrating membrane 6>/5P technology . ne0t slide7
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>ibrating shear
to prevent fouling
>/5P "echnology
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A common method to clean the membranesystem is to 1ust reverse the flow pattern:
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Membrane Processes are becomingpopular because they are considered
;9reen< technology $ no chemicals are
used in the process#
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8n the 5+ process a semi$permeable barrierallows passage of either positively charged ions
6cations7 or negatively charged ions 6anions7
while e0cluding passage of ions of the oppositecharge# "hese semi$permeable barriers are
commonly %nown as ion$e0change. ion$selective
or electrodialysis membranes#
Electrodialysis:
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,urrent re4uired in amps:
( )in out
D
in.out
F E , ,8 =
n
amp secF Faraday = C.GB(e4uivalent
, concentration in e4uivm
current efficiency 6typically B to
n = number of cells
=
=
=
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>oltage re4uired is determined by:
5 = 8 0 R
R = resistance across unit 6all cells H feedand product water7. ohms# 9enerally in
range of *& ) (& ohms#
8. in amps. as determined in previous
calculation#
5lectrode reactions:
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5lectrode reactions:
/mall amounts of hydrogen gas aregenerated at the cathode:
' ''e '- O - 6g7 'O-
+ +At the anode small amounts of
o0ygen gas are generated:
' '
$
- O '- * 'O 6g7+
+