CHEM. RES. CHINESE UNIVERSITIES 2012, 28(1), 129—132
Separation of H2O/CO2 Mixtures with Layered Adsorption
Method for Greenhouse Gas Control
XU Dong1,2, ZHANG Jun2, LI Gang2, XIAO Penny2 and ZHAI Yu-chun1*
1. School of Material and Metallurgy, Northeastern University, Shenyang 110004, P. R. China;
2. Department of Chemical Engineering, Monash University, Victoria 3800, Australia
Abstract Multiple-layered vacuum swing adsorption technique was used and investigated in order to effectively
keep the feed gas that flows into zeolite 13X zone being dry and keep the CAPEX down(not adding pre-treatment
equipment). Activated carbon fiber(ACF) and alumina CDX were laid at the lower parts of the column as pre-layers
to selectively adsorb moisture. Zeolite 13X was laid on the top of those two adsorbents as the main layer to capture
CO2. Systematic cyclic experiments show that water vapor was successfully contained within the ACF and CDX
layers at cyclic steady states. It was also found that ultimate vacuum pressure played a decisive factor for stabilizing
the water front, and achieving good CO2 purity and recovery. The findings also reveal the pathway for large-scale
CO2 capture process.
Keywords Separation; Carbon dioxide; Water vapor; Vacuum swing adsorption
Article ID 1005-9040(2012)-01-129-04
1
Introduction
The fast increasing concentrations of CO2 in atmosphere
are requiring human beings to find flexible techniques to control the emissions of greenhouse gas to atmosphere[1,2]. There
are a variety of approaches to separate CO2 from flue gas and
the most likely options include: chemical absorption, physical
adsorption, low-temperature distillation, and membranes[3,4].
Among the four approaches, CO2 capture by pressure swing
adsorption(PSA) is a promising option in terms of its relatively
low operating and capital costs. Particularly, this method shows
a great potential in CO2 capture from coal fired power stations
because of its promise in low energy consumption and system
simplicity[5].
Before CO2 is sequestered, it must be concentrated, since
the concentration of CO2 in the flue gas is typically only
10%―15%[6]. According to the literature[7―9], activated carbon
and zeolite 13X are two classical excellent adsorbents in CO2
pressure swing adsorption at a lower carbon dioxide concentration. For the capture of CO2 from dry flue gases, the simulation
researches show that CO2 can be enriched from 17% to
99.997% at a recovery of 68.4% via activated carbon[10] and
from 15% to 99% at a recovery of 53%[7]. A good technical
performance(CO2 purity>95% and recovery>80%) was
achieved by a three bed pilot-scale vacuum swing adsorption
process with 13X as adsorbent from dry gas in our group[11].
Comparison of activated carbon and zeolite 13X shows despite
high-temperature excursions, zeolite 13X is a better adsorbent
than activated carbon in non-isothermal, adiabatic PSA process
due to equilibrium selectivity[7].
However, real flue gases include water vapour(8%―12%)
and trace amounts of SOx and NOx[12,13]. Studies on CO2 separation from real flue gas show that the working capacity of
zeolite 13X dropped greatly on processing flue gas with high
absolute humidity level[14]. Conventional approaches using a
pre-treatment/drying apparatus to remove moisture from
post-combustion flue gas would considerably increase the
overall capture cost[15].
In order to keep zeolite 13X always active, a triple-layered
vacuum swing adsorption process was proposed in this study.
Multiple adsorbent layering in an adsorption bed is a well established industry technique used in hydrogen purification, air
separation and natural gas separation plants[16,17]. As for the
desiccant selection, Activated carbon fibre(ACF) and activated
alumina CDX were considered because of their great water
adsorption capacity. In this study, a shallow ACF layer was
loaded at the bottom of the bed, because ACF can adsorb considerable moisture at relatively high pressure and water desorption is easier on ACF under lower vacuum conditions. The
reason of a thin layer of ACF placement in stead of whole
pre-layer package is that too much ACF can cause serious
pressure drop in the bed and the pressure drop is detrimental to
CO2 capture performance. The proprietary alumina-based adsorbent alumina CDX was laid at the middle of the bed to
mainly adsorb/desorb water and manage the water front
movement and protect the major layer, which was comprised of
zeolite 13X. Activated alumina on the other hand is a common
adsorbent applied in industries for removing moisture in PSA
and other separation processes[18]. As there is around 10%―
15% CO2 in real flue gas, the feed gas CO2 volume fraction
———————————
*Corresponding author. E-mail: [email protected]
Received January 4, 2011; accepted September 22, 2011.
Supported by the National Natural Science Foundation of China(No.51074205) and the Fund of Corporate Research Centre
for Greenhouse Gas Technology, Australia.
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CHEM. RES. CHINESE UNIVERSITIES
was fixed at 10.5% or 15.5% respectively in this study.
2
2.1
Experimental
Adsorbents and Isotherm Measurement
The adsorption isotherm data for CO2 and N2 on zeolite
13X , CDX and ACF were taken on an ASAP 2010 Gas Adsorption Analyzer(Micromeritics, USA) at different temperatures over a pressure range of 0―118 kPa. The adsorption isotherm data for water vapour on those three materials at p/p0
from 0 to 1 were obtained on an IGA-002 Intelligent Gravimetric Analyzer system(Hiden Isochema, Ltd., UK). The physical
properties of the three materials are listed in Table 1.
Table 1 Physical properties of zeolite 13X, CDX and ACF
Adsorbent
Chemical description
Shape of adsorbent
13X
NaX
CDX
Alumina
ACF
Carbon
Cloth
Spherical
Spherical
Diameter/mm
2.0
2.0
―
Total pore vol./(cm3·g–1)
0.27
0.48
0.62
BET surface area/(m2·g–1)
445.5
441.5
1200
2.2
Experimental Set-up
The adsorption bed was made of a stainless steel column
with an effective working length of 560 mm, an ID of 49 mm
and a wall thickness of 3 mm. Along the length of the bed，ten
T-type thermocouples were inserted into the column at the positions of 60, 100, 140, 180, 220, 260, 300, 387, 474, and 560
mm from the bottom of the bed to measure the temperature
profile during the cycles. Thermocouples were also installed at
locations in the pipeline to monitor the process fluids entering
and leaving the bed. Dry compressed air was humidified in a
water bubbler, and the humidity level was simply controlled by
air pressure and water temperature. Vacuum was achieved
through an oil rotary vane vacuum pump. Various instruments
for measuring humidity, pressure, and CO2 purity were
equipped. Process control and data acquisition were realized by
Advantech Data Acquisition and Control System.
2.3
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for investigating the adsorption/desorption capacities of water
vapour and CO2 component.
We emphasize that this is not a cycle that would be used in
commercial CO2 capture plants since the latter would involve
additional steps such as pressure equalization, product purge,
etc.
3
3.1
Results and Discussion
Equilibrium Isotherms
Working capacity and selectivity are two major criteria for
adsorbent selection[19]. As is shown in Fig.2(A), the isotherms
of CO2 on zeolite 13X at different temperatures are of type I.
Even at room temperature, the pure CO2 adsorption amount can
be at around 4 mol/kg, indicating the great working capacity.
The quite low N2 isotherm curves reveal a high selectivity between CO2 and N2. However, it should be noticed from
Fig.3(A) that water isotherm on zeolite 13X is of Type II and
still retains a substantial working capacity even under vacuum
conditions. What is even worse is that water desorption on
zeolite 13X is quite hard even at relatively low pressure due to
the strong interaction between Na+ cations and polar water
molecules[20,21].
Fig.2(B) depicts CO2 and N2 isotherms onto ACF and
CDX. Both the materials can still adsorb a certain amount of
CO2 and less N2 although it is not as good as zeolite 13X for
CO2 capture. Nevertheless, what is significantly focused depends on its isotherm about moisture, as is seen in Fig.3(B).
The adsorption isotherm of water on alumina CDX is of
the“BET” shape, a typical type II isotherm. This suggests that
PSA Cycle Design
In this experiment, the temperatures for the feed gas and
column were all controlled at 30 °C. A simple single column
and three-step PSA cycle that includes only adsorption step,
desorption step and repressurization step(Fig.1) was designed
Fig.2
Fig.1
Pressure swing adsorption cycle design
and process steps
Adsorption isotherms of CO2 and N2 on
zeolite 13X(A), ACF and CDX(B) at different temperatures
(A) ■ CO2, 20 °C; * CO2, 40 °C; △ CO2, 90 °C; ▼ N2, 20 °C; ◇ N2,
40 °C; × N2, 90 °C. (B) ■ CO2, 0 °C, ACF; ● CO2, 40 °C, ACF;
★ CO2, 0 °C, CDX; ▼ CO2, 40 °C, CDX; ◇ N2, 0 °C, ACF; ◁ N2,
40 °C, ACF; ☆ N2, 0 °C, CDX.
No.1
XU Dong et al.
131
Fig.4
Fig.3
Adsorption isotherms of H2O on zeolite 13X(A),
ACF and CDX(B) at different temperatures
(A) ▼ Adsorption, △ desorption, 25 °C, p0=3.166 kPa;
(B) ■ H2O on ACF; □ H2O on CDX.
there is a steady and quick increase of water loading at high
humidity pressure. Meanwhile, a moderate hysteresis loop was
also observed, indicating the existence of mesopores in alumina
CDX, which has been confirmed by the results of liquid nitrogen adsorption. Moreover, water isotherm on ACF shows a
considerable water loading of more than 28 mol/kg at a water
partial pressure of over 1.5 kPa. However, that capacity is even
lower than that of CDX in the pressure range less than 1.0 kPa.
Meanwhile, pressure gets down to a quite low value during
evacuation step. Consequently, ACF may perform a better desiccant to adsorb more moisture compared with CDX. However,
ACF can cause significant pressure drop if there is too much
ACF packaged inside the column. So we used triple-layered
PSA for CO2 capture in this study.
3.2
Temperature profiles within one cycle at cyclic
steady state at positions of ACF(a), CDX(b)
and zeolite 13X(c) loading zone, respectively
dimensionless column length is shown in Fig.5 after the apparatus running for over 15 h. Because of the endothermic desorption from the last cycle, the temperatures were relatively
low throughout the column at the first part of feeding step.
However, temperatures rose quickly to the highest level at the
end of feed time that was caused by the CO2 adsorption at
main-layer zone and H2O adsorption at pre-layer zone, respectively. Clearly, a high temperature spot can be observed at the
bottom of the bed, which is obviously corresponding to the
water front in the adsorption bed. Then it is concluded that
water vapor has been stopped at the pre-layer zone. As the
process continued to be evacuation step(45―150 s), temperature dropped gradually to the lowest point caused by the CO2
and H2O desorption. It is worthy of mentioning that the most
dropped temperature happened in the ACF pre-layer zone,
which means water in ACF can be mostly desorbed.
Temperature Migration
The temperature profiles were observed at three important
positions along the entire bed, which were exactly at the positions of the loading zone of ACF, CDX and zeolite 13X respectively, as is shown in Fig.4. The positions were accordingly
given by thermocouples T2, T4 and T6(at Z=100, 180 and 260
mm from the feed part). It is clear that ACF zone indicated the
greatest temperature excursion due to its strong water adsorption capacity. On the other hand, CDX zone showed an intermediate temperature change, but zeolite 13X zone illustrated
the least wave fluctuation. That is because water adsorption
heat is much greater than that of carbon dioxide[15]. There was
not huge temperature excursion at zeolite 13X zone, which
indicated water vapor was successfully kept inside the
pre-layers.
The thermal profile contour plot of cycle time vs. whole
Fig.5
3.3
3D contour plot of cycle time and whole
dimensionless column length
Water Evolution
When the cycle reaches steady state, water movement
history at the outside bottom of the column is described in
Fig.6. At the first 45 s of adsorption step, water increased
sharply to around 5.4% and then the value dropped drastically
to about 1.0% at the following evacuation step. Such a trend
lasted until the end of evacuation step. During the last 3-second
repressurization step, the water concentration climbed up to the
same value as that of the previous feeding step, then a whole
cycle finished. In the next cycle, there would be the same water
evolution trend as shown in Fig.6. Fig.6 shows moisture from
flue gas could be easily moved from the adsorption bed.
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CHEM. RES. CHINESE UNIVERSITIES
Fig.6
3.4
Water concentration swing at the bottom of
adsorption bed during a cycle at cyclic steady
state
Comparison of Dry to Wet Flue Gases
In order to understand the influence of moisture on CO2
capture performance, CO2 purity and recovery from the dry and
wet gases were compared in Fig.7. Whatever the feed CO2 was
10.5% or 15.5%, the capture performance did not drop seriously. It is certainly that the CO2 purity was higher in 15.5% CO2
feed than that in 10.5% CO2 feed. So the double ACF and CDX
layers have stopped the water front further moving.
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and recoveries decreased gradually. And even worse, when feed
CO2 was at 15.5%, CO2 purity dropped greatly from 80% at 3.0
kPa to 73% at 4.2 kPa. Actually, the relationship between recovery, purity and vacuum level is strongly dependent on the
shape of the adsorption isotherm. Fig.2 shows that the slope of
the adsorption isotherm on zeolite 13X is very large, then a
lower vacuum level is required if we want to get good CO2
purity and recovery. On the other point, lower vacuum pressure
guarantees that water vapor is mostly desorbed and kept inside
the pre-layer.
Experimental studies were conducted to examine the feasibility of triple-layered vacuum swing adsorption in CO2 capture and water vapor removal from flue gas simultaneously.
Results show ACF, even only with a trace amount, together
with alumina CDX performed well to keep moisture away from
the main zeolite 13X layer. In addition, Evacuation pressure
was significant to govern the system performance. The best
CO2 purity and recovery were 74.2% and 65.7% for 10.5% CO2
feed, 80.15% and 74.1% for 15.5% CO2 feed, respectively.
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3.5
Comparison of dry(a,c) and wet(b,d) flue gases as
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