membranes & membrane process

7/23/2019 Membranes & Membrane Process

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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+

+

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