International Conference on Chemistry, Biomedical and Environment Engineering (ICCBEE'14) Oct 7-8, 2014 Antalya (Turkey)
Separation of Olefin/Paraffin Binary Gas
Mixture through Hollow Fiber Gas-Liquid
Membrane Contactor
Nayef Ghasem*, Mohamed Al-Marzouqi, Zahoor Ismail

Recent studies show that membrane separation is an
alternative, less-expensive separation method [4-6]. Using gas
liquid membrane contactor to separate ethylene from ethane
and silver nitrate as carrier solution provide promising results
[7, 8].
The target of this work was to carry further examination on
in-house fabricated PVDF hollow fiber membranes. The fiber
performances in separating propylene from its mixture with
propane was studied, accompanied with the study of a
purchased polypropylene HFM contactors. Besides, in a
previous work, it was reported that the commercially available
poly (tetrafluoroethylene-co-perfluorinated alkyl vinyl ether)
hollow fiber membranes (PFA) has demonstrated a promising
separation performance in removing CO2 and H2S
simultaneously, from their pressurized mixture with CH4 [9,
10]. PFA fibers found to be superior to the more common
ePTFE fibers, which was attributed to the smaller dimensions,
higher porosity, higher hydrophobicity and higher liquid mass
transfer coefficient of the PFA fibers. Hence, another
objective of this work was to evaluate the performance of the
PFA hollow fibers in both separations of the ethylene and
propylene from their mixtures with paraffin.
Ethylene/Ethane and Propylene/Propane separations were
experimentally studied and presented as a function of gas and
liquid flow rates, silver nitrate concentration, and propylene
concentration in feed gas using different hollow fiber
membrane contactors (HFM).
Abstract—The purpose of this work is to study the absorption
performance of a home-made polyvinylidene fluoride (PVDF hollow
fiber membrane fabricated using thermally induced phase separation
method. Separation of propylene from an propylene-propane gas
mixture, using a hollow fiber membrane system with aqueous silver
nitrate as absorption solvent, and to improve separation procedure
with a purchased polypropylene hollow fiber membrane module used
for stripping purposes. Experiments on the absorption of propylene
from propylene-propane mixture, to silver nitrate solutions were
performed. The effects of initial propylene concentration in feed gas,
silver nitrate concentration in the inlet solvent, liquid and gas flow
rates were investigated. Results reveal that propylene/propane
separation factor increases with silver nitrate concentration and liquid
flow rates. A two dimensional mathematical model was developed
for this purpose. Model predictions were validated with experimental
data. Model predictions were in good agreement with experimental
results.
Keywords—Gas
Facilitated transport
separation;
olefin/paraffin,
Silver
nitrate;
I. INTRODUCTION
Separation of light olefin from paraffin having the same
carbon number is one of the most energy intensive separations
in petrochemical processes, which uses very long distillation
columns with very high number of trays. This is attributed to
the very small differences in the relative volatilities. Light
olefins such as propylene and propylene are produced in huge
quantities in petrochemical plants. Separation of light olefin
from paraffin having the same carbon number is one of the
most energy intensive separations in petrochemical processes,
which uses distillation columns up to 100m tall and containing
over 100 trays due to the very small differences in the relative
volatilities and very large reflux ratios and also due to the
need for sub-ambient temperatures [1]. These olefins are
generally produced by dehydrogenation of the matching
alkanes. Their separation is difficult process because of the
similarity in molecular sizes and physical properties [2].
Separation of olefin/paraffin is important in the petrochemical
industry [3]. Traditional systems, like distillation, other
methods such as extractive distillation and absorption are also
expensive and consuming energy; studies show that these
methods have no improvement over traditional distillation.
II. EXPERIMENTAL
A.
B. Experimental Setup
To compare the olefin/paraffin separation performance of
the three materials of hollow fiber membranes; PVDF, PP, and
PFA, the experimental setup shown in Fig.2.1 was used. The
system utilizes hollow fiber membrane contactors for under
absorption/desorption principle; absorption of olefins from
their olefin/paraffin binary mixtures by chemical
complexation with silver nitrate solution, and regeneration of
the absorbing liquid.
Nayef Ghasem*, Mohamed Al-Marzouqi, Zahoor Ismail, are with
Department of Chemical & Petroleum Engineering, UAE University, Alain
city, UAE, *Email address: [email protected], Tel: + 971 3 713 5313, Fax: +
971 713 4962
http://dx.doi.org/10.17758/IAAST.A1014017
Reagents and Materials
PFA fibers purchased from Entegris, Germany. Polypropylene
HFM contactors G543, purchased from Membrana, USA. Silver
nitrate, received from Sigma Aldrich, Germany. Ethylene (99.95%),
Ethane (99.5%), Propylene (99.5%), and Propane (99.5%), gas
cylinders purchased from Air Products, UAE. Nitrogen (99.99%) gas
cylinder received from Sharjah Oxygen (UAE).
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International Conference on Chemistry, Biomedical and Environment Engineering (ICCBEE'14) Oct 7-8, 2014 Antalya (Turkey)
In addition to the commercially available PP HFM
contactors (G543), HFM contactors were fabricated of the
PFA and the PVDF fibers. Fibers were installed in a Perspex
tubes with 7.9 mm ID and 12.1 mm OD, details are in Table
2.1. The HFM contactors were employed in the system Fig.
2.1. Olefin/paraffin gas mixture fed to the shell-side of the
absorption contactor. Silver nitrate aqueous solution pumped
to lumen-sides of both the absorption and desorption
contactors. Nitrogen gas of 5L.min-1 used as sweep gas in the
shell-side of the desorption module. Gas compositions were
monitored using X-Stream XE, Emerson gas analyzers. The
dryness of gas sent to the analyzer was ensured using CaSO4
gas dryer. Experiments carried out at room temperature, and
atmospheric pressure.
While, in regard of the effect of the silver nitrate
concentration, Figure 3 presents the membranes performance.
Silver nitrate concentration was increased from 1M to 3M.
Performances were evaluated under conditions of 50%
propylene in feed gas composition and 300 mL/min gas flow
rate. Although, experiments carried out over range of liquid
flow rate from 10 mL/min to 50 mL/min, those of 50mL/min
absorbent flow rate are only presented in Figure 2 for the
matter of comparison. Increasing silver nitrate concentration
enhanced absorption values, yet the observed nonlinear
dependence of the absorption values on silver nitrate
concentration, can be attributed to the used membrane-based
solution regeneration technique, where increasing silver
nitrate concentration increases absorption values thus,
increase the load on the desorption part of the system, which
is considered the limiting factor. It still important to notice
that, PVDF membranes had the highest absorption flux
among.
Fig. 1 Experimental setup used in separation process[1].
TABLE 2.1
SPECIFICATIONS OF THE HOLLOW FIBER MEMBRANE MODULES
Membrane Material
Inner Diameter, mm
Outer Diameter, mm
Effective membrane length,
cm
No. of fibers
Inner surface area, m
Porosity, %
III.
2
G543
PP
0.22
0.30
28% PVDF
PVDF
PFA
24.0
141
0.25
0.65
10
30
22.5
2300
20
56
0.16
0.00792
0.00989
45
56.57
56.8
Fig. 2 Propylene absorption flux as function of liquid flow rate
. RESULTS AND DISCUSSIONS
A. Propylene absorption and absorbent’s concentration and
flow rate
Figure 2 shows how the different types of hollow fiber
membranes; PVDF, PP, and PFA had worked with the change
in the absorbent’s flow rate, and how is the relation between
the propylene absorption flux and absorbent’s flow rate.
Experiments were carried out with 2M silver nitrate, 300
mL/min gas flow rate and 50% propylene concentration in
feed gas. Increasing absorbent’s Flow rate brings in more
silver ions, which as result should enhance the membranes
performances in propylene separation. This can be clearly
seen in the propylene absorption flux values of the PVDF
fibers, where it was enhanced significantly, though it was less
pronounce in that of the PP, and PFA membranes. Behavior
http://dx.doi.org/10.17758/IAAST.A1014017
Fig. 3 Propylene absorption flux as function of silver nitrate
concentration
B. Propylene absorption and Feed’s Gas initial
composition, and flow rate
Figure 4 shows propylene absorption fluxes for the three
membranes over a propylene concentration of 50% to 80% in
initial feed gas. It was expected that increasing propylene
concentration in feed gas, will introduce higher amount of
propylene available for silver/propylene complexation, while
beyond a certain concentration, the silver ions becomes less
efficient on a per mole basis complexing with the olefin due to
the limitations in silver ions available for the complexation,
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International Conference on Chemistry, Biomedical and Environment Engineering (ICCBEE'14) Oct 7-8, 2014 Antalya (Turkey)
porosity. While in most of results, PP membranes showed
higher absorption values than that of PFA, which can be
explained by the lower inner diameter of PP fibers to that of
PFA. Similar trend was reported by Rajabzadeh et.al. [11] as
the membrane inner diameter decreases, the liquid velocity in
the tube increases, which results in a higher mass transfer
coefficient.
and as result propylene absorption values will decrease. As
was expected, when the feed fraction of propylene increases
from 50% to 70% an increase in propylene absorption values
can be observed where it reaches a maximum followed by a
decline in flux values for that of PVDF and PP fibers in
contrast to that of PFA fibers where absorption fluxes
continued increasing up to the 80% propylene.
Fig 5 presents propylene absorption fluxes for all
membranes as a function of feed gas flow rate. Since
increasing gas flow rate lowers the residence time of the
propylene, this ends up with decrease in overall absorption
values for all membranes.
D. Ethylene Absorption with PFA fibers
Two modules were fabricated using PFA fibers. Details of
these modules are given in Table 1. The experimental
evaluation of the ethylene/ethane separation was carried out at
different; silver nitrate concentrations, liquid flow rates, feed
gas compositions, and gas flow rates.
E. Effect of silver nitrate concentration and flow rate
The effect of silver nitrate concentration on ethylene
absorption flux at different liquid flow rates with a fixed gas
flow rate of 300 mL.min−1 was investigated Figure 6. It was
evident that the flux increased with increasing the silver
nitrate concentration and liquid flow rate. However, the
leveling off, which was pronounced at fixed silver nitrate
concentration and different liquid flow rates could be
attributed to the high ethylene absorption and hence higher
load on the stripping or the desorption module, therefore no
further enhancement in the ethylene separation can be
predicted. Moreover, at further silver nitrate concentrations
(higher than 2M) ethylene absorption was decreasing with
increasing liquid flow rate, and this was also attributed to the
lower desorption efficiency with the higher ethylene
absorption.
Fig. 4 Propylene absorption flux as function of propylene
concentration in feed gas
Fig. 6 Ethylene absorption flux as function of silver nitrate
concentration and flow rate, using PFA fibers
Fig. 5 Propylene absorption flux as function of feed gas flow rate
F. Effect of initial ethylene concentration and feed gas flow
rate
C. Performance of the PVDF, PP and PFA
Since the absorbent liquid flows in the tube side, it is
considered that the inner surface is closely related to the gas–
liquid contacting area. Both porosity and Inner diameter of
hollow membrane fibers affects the performance of fibers.
Inner diameter of the membranes is in the order of PP(0.22
mm) > PFA (0.25 mm) > PVDF (0.42 mm) while membrane’s
porosity is in the order of PP (45 %) > PVDF (56.57%) > PFA
(56.8%). This explains the higher propylene absorption flux
values of PVDF fibers to that of PP, where PVDF has higher
http://dx.doi.org/10.17758/IAAST.A1014017
The effects of the initial ethylene concentration in feed gas
on absorption flux values are shown in Figure 7. Although
ethylene absorption flux increased with increasing initial
ethylene concentration, it has decreased at the 80%. Likewise,
this is attributed to limited amount of silver ions available for
complexation at the high initial ethylene concentration in the
feed gas. The effect of gas flow rate on the ethylene separation
flux was also investigated. Similar trends of that of propylene
absorption were observed, for a given liquid flow rate,
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International Conference on Chemistry, Biomedical and Environment Engineering (ICCBEE'14) Oct 7-8, 2014 Antalya (Turkey)
[2]
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ethylene separation flux decreased with increasing gas flow
rate Figure 8.
Fig. 7 Ethylene absorption flux as function of initial ethylene
concentration, using PFA fibers.
Fig. 8 Ethylene absorption flux as function of gas flow rate, using
PFA fibers.
IV. CONCLUSION
The separation of olefin/paraffin via in house made PVDF
hollow fiber membrane is employed in ethylene and propylene
separation from an ethylene-ethane and propylene/propane gas
mixtures. The PVDF constructed membrane contactor module
is utilized in the absorption–desorption system with various
concentration of aqueous silver nitrate concentrations as a
stripping solvent. An acquired polypropylene hollow fiber
membrane module with high surface area was used to improve
stripping method. A variety of experiments on the absorption
of olefin from olefin-paraffin gas mixture, into silver nitrate
solutions were performed. The effects of initial olefin
concentration in feed gas, silver nitrate concentration, and the
gas flow rate effects on separation performance were
investigated. Results reveal that as silver nitrate concentration
and liquid flow rate increases removal flux and percent
separation of ethylene increases.
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R.B. Eldridge, Olefin/paraffin separation technology: a review, Ind.
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2208–2212.
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http://dx.doi.org/10.17758/IAAST.A1014017
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