J. Energy Power Sources
Vol. 1, No. 5, 2014, pp. 265-269
Received: August 30, 2014, Published: November 30, 2014
Journal of Energy
and Power Sources
www.ethanpublishing.com
Gas Permeation on Silica Membrane for Lactic Acid
Esterification Applications
Edidiong Okon, Mohammed Nasir Kajama and Edward Gobina
Center for Process Integration and Membrane Technology (CPIMT), School of Engineering, Robert Gordon University, Aberdeen,
AB10 7GJ, United Kingdom
Corresponding author: Prof. Edward Gobina ([email protected])
Abstract: Membrane separation processes is receiving an increasing attention in several separation process including VOC recovery
and separation from natural gas as well as esterification reactions of lactic acid to produce ethyl lactate which is an environmental
friendly solvent. Inorganic ceramic membrane can enhance the reaction yield in the esterification reaction by selectively removing
water from the reaction product in separation processes. In this paper, the performance characteristics of inorganic ceramic membrane
with single gas was carried out to determine the permeation transport of single gases through porous membrane at the temperature of
298-723 K and gauge pressure range of 0.10 to 1.00 bar. The single gases used for the investigation include: He (Helium), N2
(Nitrogen), CH4 (Methane) Ar (Argon), CO2 (Carbon Dioxide) and H2 (Hydrogen) gas. The surface morphology and structure of the
membrane pore size distribution was examined using scanning electron microscopy coupled with energy dispersive x-ray analyser
(SEM-EDXA). The gas flux was found to increase with respect to feed pressure in the order of Ar > He > CO2 > N2 indicating Knudsen
mechanism of transport. The gas permeance was found to decrease with increase in gauge pressure.
Keywords: Permeance, gas transport mechanism, esterification, VOC recovery, inorganic ceramic membrane.
1. Introduction
Membrane technology has been successfully
employed over the years in several industrial
applications and has also replaced traditional,
environmentally polluting and energy-demanding
separation techniques. Membrane filtration processes
based on RO (Reverse Osmosis) system together with
UF (Ultrafiltration) and MF (Microfiltration) units have
been introduced in waste-water production plants that
manufacture water suitable for both industrial use and
potable quality [1]. Moreover, membrane-based
separation has attracted important attention for CO2
capture applications due its numerous advantages
including lower operating cost and high energy savings.
Carbon dioxide separation from other gases is critically
significant for industrial applications including biogas
purification and capture of CO2 from advanced
fossil-based power generation sources [1]. The use of
inorganic ceramic membrane to selectively eliminate
water from the reaction product during esterification of
lactic acid is yet another important application that has
attracted a lot of attention [2]. Esterification reactions
are said to be usually limited by equilibrium and as such
do not reach completion [3]. However, using a
membrane can result in higher conversion by shifting
the chemical equilibrium towards the formation of the
product by removal of water in the reaction mixture
using methods such as membrane process and reactive
distillation [4]. Inorganic Porous ceramic membrane
have shown a lot of advantages in separation process
due to their high mechanical, thermal robustness and
chemical stability and are therefore useful candidates
for high temperature membrane reactor applications.
Although these membranes possess several advantages,
the major drawback of porous ceramic membrane is the
low selectivity offered by mostly mesoporous materials
266
Gas Perm
meation on Sillica Membrane
e for Lactic Acid
A
Esterifica
ation Applicatiions
for gas-separrations [5]. Thhe separation efficiency annd
flux depend on the micrrostructural features
f
of thhe
membrane inncluding thicknness and poro
osity as well aas
pore size andd its distribution [6].The classification foor
the membranee pore structurre are explaineed based on thhe
IUPAC definnition of poore classificattion in whicch
micropores coorresponds to the pores smaaller than 2 nm
m,
In thhis study, a commercial mesoporous ceramic
supporrt was prepareed. Membranee was synthessized via
dip-coaating method. The membrrane was inveestigated
with siingle gas perm
meation experriments at 0.10 to 1.0
barg annd 298 to 723 K.
macropores foor pores greateer than 50 nm and mesoporees
are typically in the range of 2-50 nm [7]. The most
common matterials for cerramic membraanes are ZrO
O2,
Al2O3, SiO2 and
a TiO2 [5]. Gas transportt across porouus
membrane is controlled byy different ph
henomena, annd
various modeels have beenn developed to explain thhe
The preparation of
o the membrane was carrried out
using the dip-coating method and
a
the suppport was
coated once. This was perfformed in a clean
environnment to avoiid the formatioon of pin-holees on the
thin sillica film [12]. The outer and inner pore rradius of
the suppport was 10 and 7 mm reespectively, w
while the
total length
l
of th
he support was
w
36.6 cm
m. The
modification was acchieved usingg a similar meethod as
that off Gobina 2006
6 [13]. The gas permeation test was
carriedd out using diffferent gases including argon (Ar),
N2 (N
Nitrogen), He (Helium), H2 (Hydrogenn), CH4
(Methaane) and CO2 (Carbon Diioxide) at thee gauge
pressurre range of 0.10-1.00
0
bar and the tempperature
range of
o 298-723 K. The gas perm
meation was m
measured
using a digital flo
ow meter. Membrane
M
arrea was
calculaated using the formula:
mass transpport. Howevver, the maajor transpoort
mechanisms include; surrface flow, viscous flow
w,
multilayer adssorption, moleecular sieving, Knudsen flow
w
and capillary condensationn [8]. Howeveer, of the threee
major mechaanisms (i.e m
molecular siev
ving, Knudseen
diffusion and surface diffussion), only mo
olecular sievinng
c
offer seeparation effiiciencies greaat
mechanism can
enough to be commerciallyy attractive. In order to obtaiin
high separatiion efficiencyy using any
y of the threee
mechanisms, close control of pore size must
m be coupleed
with a high pore
p
density [[9]. Mesoporo
ous membranees
occur withinn a transitionn region betw
ween Knudseen
diffusion annd viscous flow and the dominannt
mechanism iss extremely ddependent on the permeatinng
molecule andd the pore size whereass, microporouus
membranes basically
b
obeyy an activateed transport oor
molecular sieeving mechaniism [5]. Ceramic membranne
can be preparraed using diffferent modificcation methodds
including CV
VD (Chemical Vapour Depo
osition), sol-geel
and sintering processes [100]. However, sol-gel methood
are usually veery attractive bbecause they arre close controol
of pore micrrostructure thhat is directly
y applicable tto
cheap and sim
mple membranne fabrication, by employinng
either spin or dip-coating procedure [9
9]. The sol-geel
g a mesoporouus
modification method involvve dip-coating
s
polymerrs
membrane inn a mixture coonsisting of silica
derived from
m alkoxide com
mpounds (soll), followed bby
controlled dryying and firingg at higher tem
mperatures [11].
2. Meethods
A=
πL r1 −rr2
In
r1
r2
(1)
where A = membraane area (m2),
) L = lengthh of the
membrrane (m), r1 = membrane outer
o
radius (m
mm), r2
= mem
mbrane inner raadius (mm) annd π = 3.1422 [14].
The characterization of the porre size distribbution of
the cerramic supporrt was carriedd out using sscanning
electroon microscopy
y fused with ennergy dispersive x-ray
analyseer (SEM-EDX
XA). Single gases
g
were puurchased
from BOC,
B
UK. Fig. 1 shows the gas permeatioon setup.
Fig. 1 Gas permeatio
on experimentaal setup.
Gas Perm
meation on Sillica Membrane
e for Lactic Acid
A
Esterifica
ation Applicatiions
3. Results and
a Discusssion
The innerr, outer annd cross-secttional surfacce
morphologiess of the suppport and membrane werre
analysed by scanning ellectron micro
oscopy (SEM
M)
(Zeiss EVO LS10).
L
The suupport was fou
und to be defecct
free. Figs. 2-33 show SEM iimages of the inner and outeer
surface beforre modificatioon. The SEM
M of the crosss
section of thee support is shhown in Fig. 4.
4
Dip-coating parameters such as visco
osity of the sool,
immersion annd drying timee and also dipp
ping speed arre
Fig. 2 SEM im
mage of the sup
pport inner surrface.
267
the factors that affecct both the mem
mbrane thicknness and
its poree size distribu
ution. The fluxx (mol m-2 s-1) of CO2,
Ar, N2 and He were plotted againnst the gauge ppressure
(bar) and
a temperatu
ure to investiggate the behavviour of
the gasses through thee dip-coated membrane.
m
Acccording
to Walll et al. (2010)), the Knudsenn flux dependds on the
molecuular weight off the permeatiing gas moleccule [8].
From Fig.
F 5, a straig
ght line graphh with high reggression
values,, a positive slo
ope and interccept was obtaained for
all the gases. The gaas flux was foound to increaase with
respectt to gauge prressure. A sim
milar result w
was also
obtaineed by Wall et al.
a (2010). Ar and He gases showed
a highher flux com
mpared to otther gas, inddicating
viscouus flow as the dominant meechanism of trransport
for the gases. The gaas flux was foound to increase in the
order of
o Ar (40 g/m
mol) > He (44 g/mol) > C
CO2 (44
g/mol) > N2 (28 g/m
mol), which is not exactly bbased on
their molecular
m
weeight values as Knudsen flux is
dependdent on the mo
olecular weighht of the diffuusing gas
molecuule.
Fig. 6 depicts th
he relation beetween flow rates of
p
tem
mperatures at 0.6 barg.
different gases and permeation
It can be
b seen that th
he behaviour of hydrogen and that
of the ternary
t
mixturre of CO2/CO
O/H2 slightly roose after
285 ℃.
℃ This is due to the influennce of hydroggen gas.
Hydroggen transport through
t
silicaa is known to bbe due to
an activvated transporrt mechanism
m. This is in coontrast to
Fig. 3 SEM im
mage of the sup
pport outer surrface.
Fig. 4 SEM im
mage of the sup
pport’s cross-seectional area.
the othher single gasees N2 and CO2 as well as thhe binary
mixturre of C3H8/N
N2 which maintained a reelatively
constannt permeation
n value for thhe entire perrmeation
temperratures indiccating a typpical behaviiour of
Knudseen diffusion mechanism
m
beehaviour.
Fig. 7 depicts the permsellectivity of helium,
methanne, nitrogen
n and hydrrogen over carbon
dioxidde. It can be seeen that the selectivity
s
of helium,
methanne and nitrog
gen over carboon dioxide deecreases
as the permeation
p
teemperature inccreases. Selecctivity of
3.1, 1.224 and 1.17 was
w obtained for He, CH4 and N2
over CO
C 2 at 298 K. Unlike
U
hydroggen (from 2988 to 438
K), thee selectivity was maintainned at 2.39 bbut from
438 to 558 K the selectivity increeased to 2.44.. Higher
268
Gas Perm
meation on Sillica Membrane
e for Lactic Acid
A
Esterifica
ation Applicatiions
Permeate Flux (mols -1 m-2 )
0.2
CO2
Ar
N2
0.18
He
y = 0.1621x + 0.0247
R² = 0.9984
0.16
0.14
0.12
y = 0.1483x + 0.0121
R² = 0.9796
y = 0.1086x + 0.01
168
R² = 0.9915
0.1
0.08
y = 0.0763x + 0.019
R² = 0.96
618
0.06
0.04
0.02
0
0
0.2
0.4
0.6
0.8
1
1.2
Gauge pressure (bar)
Fig. 5 Permeeate flux (molm
m-2s-1) against gauge pressurre
(bar) at 353 K..
Fig. 6 Flow
w rates (L/min
n) of gases ass a function oof
temperature att 0.6 barg.
Fig. 7 Permsselectivity effecct as a function
n of permeatioon
temperature att 0.85 barg.
selectivity off 2.59 was obttained at 723 K. This show
ws
that more hydrogen
h
willl be recoveered at higheer
temperature.
The permeance (molm-2s-1Pa-1) of the Ar, N2, He annd
CO2 gases weere also plotteed against the different
d
gaugge
pressure (bar)) at 353 K. It can be seen from
f
Fig. 8 thaat
the permeancee of the gases decreases witth respect to thhe
gauge pressurre. He gas seem
ms to show a bit
b of deviatioon
from the trennd at the gaugge pressure raange of 0.2-0..6
bar, but shoowed a decreease with inccreasing gaugge
pressure. It was
w suggestedd that there co
ould have beeen
Fig. 8 Gas permeancce against gaugee pressure at 3553 K.
Fig. 9 Gas pereman
nce (molm-2s-1Pa
P -1) against in
nverse of
molecullar weight at 0..80 bar and 3533 K.
another mechanism of transport that
t
is responssible for
the floow of helium
m gas througgh the silica support
membrrane at 353 K..
The graph of perm
meance (mol.m-2s-1Pa-1) in relation
to the inverse of thee square root of the gas m
molecular
weightt was also plo
otted at the gaauge pressure of 0.80
bar andd at 353 K. Frrom Fig. 9, itt was found thhe graph
did nott follow the treend of Knudseen flow mechaanism of
gas traansport which
h could have been a straiight line
graph starting
s
from the
t origin. Thhis indicates thhat there
could have been an
nother flow mechanism thhat was
responnsible for the flow
f
of the gasses.
4. Con
nclusions
The effect of supp
port modification on the sinngle gas
transpoort characteriistics throughh inorganic ceramic
membrrane was achieved using Knudsen
K
and viscous
mechannisms of tran
nsport. The gaas flux was ffound to
increasse with respeect to gaugee pressure inndicating
viscous flow mech
hanism. Howeever, the perrmeance
relationn with the inverse squaare root of the gas
Gas Permeation on Silica Membrane for Lactic Acid Esterification Applications
molecular weight did not give a straight line graph
suggesting another mechanism of gas transport. The
SEM images of the membrane showed a defect-free
surface. The gas permeance decreases with increase in
gauge pressure. The selectivity of He, CH4 and N2 over
CO2 gas decreases with increase in temperature. Higher
selectivity was obtained for hydrogen gas indicating
higher recovery of hydrogen gas at higher temperature.
The selectivity of helium, methane and nitrogen over
carbon dioxide decreases as the permeation
temperature increases.
Acknowledgment
The Authors acknowledge the Center for Process
Integration and Membrane Technology (CPIMT),
School of Engineering, RGU for providing the
membrane and also the School of Pharmacy and Life
Sciences RGU for the SEM images.
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