ADSORPTION OF CARBON DIOXIDE ON ALKALI METAL
EXCHANGED
ZEOLITES
Emilio Muñoz, Eva Díaz, Salvador Ordóñez, Aurelio Vega
University of Oviedo, Oviedo, Spain
Introduction
The increasing atmospheric CO2 concentration, mainly caused by fossil fuel
combustion, have become an important concern for global warming because the atmospheric
CO2 concentrations increased significantly in the last century and rises continuously at a
faster rate. Carbon dioxide is produced in large quantities by many important industries such
as fossil-fuel-fired power plants, steel production, chemical and petrochemical manufacturing,
cement production, and natural gas purification. The reasons for the CO2
removal are
traditionally technical and economical concerns. Carbon dioxide present in natural gas will
reduce the heating value of the gas and as an acid component it has the potential to cause
corrosion in pipes and process equipment and also to cause catalyst poisoning in ammonia
synthesis (1). In the past decades, CO2 removal from flue gas streams started
as a
potentially economic source of CO2, mainly for enhanced oil recovery
operations. Moreover,
CO2 was also produced for other industrial applications such as carbonation of brine, welding
as an inert gas, food and beverage carbonation, dry ice, urea production, and soda ash
industry (2). However, environmental concerns, such as the global climate change, are now
focused as one of the most important and challenging environmental issues facing the world
community, and have motivated intensive research on CO2 capture and
sequestration.
Carbon dioxide as one of the greenhouse gases (GHG) is currently responsible for over 60 %
of the enhanced greenhouse effect, methane (CH4) contributes 20 %, and the
remaining 20
% is caused by nitrous oxide (N2O), a number of industrial gases, and ozone. Scientific
evidence now strongly suggests that increased levels of GHG may lead to higher
temperature, and cause climate change on a global scale. Various climate models estimate
that the global average temperature may rise by about 1.4 – 5.8 ºC by the year
2100 (3).
The standard method to removal CO2 break down the whole system into its
component parts: capture, transport, and storage. The capture and ulterior storage in a
geologic reservoir is, nowadays, the technique more useful to reduce the CO2
concentration
in the atmosphere. However, the total amount of antrophogenic carbon dioxide that is
captured is lower than 19 Mt/year. The reason is because the capture is only possible for
large stationary sources of CO2, like power plants or cement fabrics. The large stationary
sources produce around 60 % of the total carbon dioxide, which is 14 Gt CO2/year.
Below, each of these components is defined:
Capture, is the production of a CO2 stream that is ready for transport and
storage. CO2
from large industrial sources is usually part of a stream composed of several gases. In
general, the CO2 is separated as a fairly pure stream (90-99% pure) and then compressed to over 100 atm. While power plants are the largest single source of CO2
(over a third of all CO2 emissions), other industrial operations (e.g., ammonia plants,
refineries, natural gas processing) also provide attractive targets. In most cases,
capture (including compression) is by far the largest cost component (typically 80% of
the costs for power plants) (4), Table 1.
Transport is moving CO2 from the capture site to the storage site. For moving large
amounts of CO2, pipeline transport is almost always the preferred mode. Small amounts of CO2 can be transported via truck, while tanker ships are being
considered
for some circumstances.
Storage is comprised of injecting CO2 into a reservoir. Monitoring and
verification fall
under this component.
Some processes (e.g. acid gas processing, hydrogen and ammonia production)
produce point sources of highly concentrated or pure CO2. The process
already includes CO2
separation therefore these sources typically only require compression and dehydration for
CO2 capture and therefore the capture cost is relatively low (4-8 ?/t CO2).
However, these
sources are typically dispersed and small scale with the total current worldwide, estimated to
be around 120 Mt/year. The power sector represents the largest opportunity for capture and
storage. In the power sector, capture using existing technologies such as post-combustion
amine systems have a current costs in the range of 32-48?/t CO2, avoided for
new build
projects using pulverised coal or natural gas combined cycle generation (2,6). Integrated
gasification combined cycle (IGCC), an emerging coal or coke-based technology for power
generation offers the lowest cost of capture for power at 12-20?/t CO2 as the
CO2 stream is
already concentrated (7).
Thus, it is evident that the fact of obtaining an economically technique to capture the
cabon dioxide is of prime concern.
Types of techniques for capture of CO2
There are three main techniques for capture of CO2 in power plants:
pre-combustion
capture, oxy-fuel combustion and post-combustion capture.
In pre-combustion capture, fuel is reacted with oxygen or air, and in some cases
steam, to give mainly carbon monoxide and hydrogen. This process is known as gasification,
partial oxidation or reforming. The mixture of mainly CO and H2 is passed
through a catalytic
reactor, called a shift converter, where the CO reacts with steam to give CO2
and more H2.
The CO2 is separated and the H2 is used as fuel in a gas turbine combined
cycle plant. The
process is, in principle, the same for coal, oil or natural gas, but when coal or oil are used
there are more stages of gas purification, to remove particles of ash, sulphur and nitrogen
compounds and other minor impurities. The CO2 concentration and pressure
are both higher
in pre-combustion capture than in post-combustion capture, so the CO2
capture equipment is
much smaller and different solvents can be used, with lower energy penalties for
regeneration.
The oxy-fuel combustion consists on increasing the concentration of CO2 in
flue gas by
using concentrated oxygen instead of air for combustion, either in a boiler or gas turbine. The
oxygen would be produced by cryogenic air separation, which is already used on a large
scale, for example in the steel industry. If fuel is burnt in pure oxygen, the flame temperature
is excessively high, so some CO2-rich flue gas would be recycled to the
combustor to make
the flame temperature similar to that in a normal air-blown combustor. The advantage of
oxygen-blown combustion is that the flue gas has a CO2 concentration of over
80%, so only
simple CO2 purification is required. Another advantage is that NOX formation is
suppressed,
and the volume of gas to be treated in the flue gas desulphurization plant is greatly reduced.
Additionally, other than a need for flue gas desulphurization, oxyfuel combustion relies mainly
on physical separation processes for O2 production and CO2 capture thereby
avoiding the
use of any reagents and/or solvents that contribute to operating costs and the environmental
disposal of any related solid or liquid wastes. The main disadvantage of oxyfuel combustion is
that a large quantity of oxygen is required, which is expensive, both in terms of capital cost
and energy consumption.
Post-combustion capture involves separating CO2 from the flue gas produced
by fuel
combustion. A variety of techniques can be used for this separation:
Absorption: is the most employed method for the removal of CO2. The most
common
solvent is monoethanolamine (MEA). Prior to CO2 removal, the flue gas is
cooled and
particulates and other impurities are removed as far as possible. It is then passed into
an absorption vessel where it comes into contact with the chemical solvent, which
absorbs much of the CO2 by chemical reactions to form a loosely bound
compound.
The CO2– rich solvent taken from the bottom of the absorber is passed into another
vessel (stripper column) where it is heated with steam to reverse the CO2
absorption
reactions. CO2 released in the stripper is compressed for transport and storage and
the CO2–free solvent is recycled to the absorption vessel. CO2 recovery rates
of
around 85-95% capture are normally proposed and product purity can be in excess of
99% (8). The main concerns with MEA and other amine solvents are corrosion in the
presence of O2 and other impurities, high solvent degradation rates from reaction with
SOX and NO2 and the large amounts of energy required for regeneration.
Membranes: gas separation membranes rely on differences in physical or
chemical
interactions between gases and a membrane material, causing one component to
pass through the membrane faster than another. Although there are various types of
membrane are currently available, any of them achieve high degrees of separation, so
multiple stages and/or recycle of one of the streams is necessary. This leads to increased complexity, energy consumption and costs. There is also a gas absorption
membranes hybrid system (9). The CO2 diffuses through the membrane and is
removed by an absorption liquid such as amine, which selectively removes certain
components. In contrast to gas separation membranes, it is the absorption liquid, not
the membrane that gives the process its selectivity.
Cryogenics: CO2 can be separated from other gases by cooling and
condensation.
Cryogenic separation is widely used commercially for purification of CO2 from
streams
that already have high CO2 concentrations (typically >90%) but it is not
normally used
for more dilute CO2 streams. A major disadvantage of cryogenic separation of CO2 is
the amount of energy required to provide the refrigeration necessary for the process,
particularly for dilute gas streams. Another disadvantage is that some components,
such as water, have to be removed before the gas stream is cooled, to avoid blockages. Cryogenic separation has the advantage that it enables direct production of
liquid CO2, which is needed for ship transport.
Solid sorbents: sorbents such as calcium or lithium based oxides can react with CO2 to
form carbonates and the carbonates can be regenerated to oxides by heating to a
higher temperature (10). These processes have the potential to reduce efficiency
penalties compared to wet absorption processes. A weak point of processes that use
natural solid sorbents (limestone and dolomite) is that they deactivate rapidly and a
large make-up flow of sorbent is needed, although the deactivated sorbent may find
application in the cement industry. Lithium based sorbents are much more durable but
they are intrinsically expensive materials.
Adsorption: some solid materials with high surface areas, such as zeolites and
activated carbon, can be used to separate CO2 from gas mixtures by
adsorption. Gas
is fed to a bed of solids that adsorbs CO2 and allows the other gases to pass
through.
When a bed becomes fully loaded with CO2, the feed gas is switched to
another clean
adsorption bed and the fully loaded bed is regenerated to remove the CO2. In
pressure
swing adsorption (PSA), the adsorbent is regenerated by reducing the pressure. In
temperature swing adsorption (TSA), the adsorbent is regenerated by raising its
temperature and in electric swing adsorption (ESA) regeneration takes place
by
passing a low-voltage electric current through the adsorbent. PSA and TSA are used commercially for gas separation and are used to some extent in hydrogen production and in removal of CO2 from natural gas, but ESA
is
poorly explored and tested at present. Adsorption is not yet considered attractive for
large-scale separation of CO2 from flue gas because the capacity and CO2
selectivity
of available adsorbents is low (11). However, it may be successful in combination with
another capture technology. Adsorbents that can operate at higher temperatures in the
presence of steam with increased capacity and improved selectivity are needed.
Activated carbons have been widely used as carbon dioxide adsorbents due to their
high surface area, which confers them high adsorption capacity. However, this high capacity
of adsorption is limited at room temperatures. Przepiorski et al. (12) have tested activated
carbons in the capture of CO2 at 25 and 36 ºC, observing an important
decrease in the
capacity of adsorption in only 9 ºC. For this reason, in this work, we have selected zeolites as
adsorbents for carbon dioxide capture. High aluminium (or low silicon) content zeolites have
been extensively used for separation of gases including carbon dioxide from gas mixtures.
Inui et al. (13) studied the relation between the properties of various zeolites and their CO2
adsorption behaviours, concluding that 13X zeolites were the most proper choice. Likewise,
Kumar et al. (14) established that NaY zeolite could be a substitute of 13X zeolite due to its
easier regenerability. Furthermore, in order to improve the capacity of adsorption of these
zeolites, treatments with Cs were carried out, since it is the most electropositive metal of the
periodic table. The effect of temperature, as well as the regenerability of these zeolites, both
after CO2 desorption and after water desorption, was tested.
Experimental section
Zeolites NaX (Alltech) and NaY (Zeolyst Corporation) are used as received. The
alkaline treatment of the zeolites was carried out at 70 ºC for 2 h, followed by drying at 100 ºC
12 h and calcination at 650 ºC for 4 h. Alkali metal solutions (0.5 M) were prepared disolving
CsOH (Avocado) or Cs2CO3 (Avocado) into distilled water. In each case, 2 g of zeolite were
suspended into 100 mL of the Cs+ solution. The modified zeolites were
recovered by filtration
and repeatedly washed with distillate water to remove the impurities completely. The resulting
zeolites were pretreated at 650 ºC in an oven for 4h in order to remove the moisture and
other contaminants prior to the experiments. Prepared zeolites will be referred to as CsA-B,
where A is the type of zeolite (X or Y) and B refers to the cesium precursor (OH
for CsOH
and c for Cs2CO3).
Adsorption experiments were carried out in a Micromeritics TPD-2900 apparatus
connected to a Glaslab 300 mass spectrometer using He as the carrier gas. Before each
TPD experiment, 50 mg of sample was introduced in a quartz tube and outgassed in a He
flow of 30 mL/min by thermal treatment at 600 ºC for 1 hour, with a heating rate of 10 ºC/min
from room temperature. After being cooled to 50 ºC, the adsorbent material was contacted
with the gaseous feed (pure CO2) for 20 min. The reversibly adsorbed carbon
dioxide was the
removed by treatment of the sample in He flow for 1 h at 50 ºC. The completion of this
desorption process was confirmed by the recovery of the baseline of the mass spectrometer.
The TPD tests were carried out by heating the sample with a ramp of 10 ºC/min between 50
ºC and 600 ºC with constant He flow. In order to study the regenerability of the adsorbents,
after keeping the latter temperature constant for 60 min, the sample is cooled to 50 ºC and
the adsorption process repeated. The selectivity for CO2 adsorption in
presence of water
vapour is studied saturating the sample at 50 ºC with water, by successive injection of water
pulses, and then the desorption process is carried out according to the previous described
method. Once the sample is cooled to 50 ºC, it is saturated with CO2 in order to
evaluate its
adsorption after the water adsorption.
Nitrogen adsorption-desorption isotherms were obtained at –196 ºC on a
Micromeritics
ASAP 2000 instrument. Previously, the samples were outgassed at 200 ºC for 6 h in high
vacuum. Acidity strength studies were carried out by NH3-TPD, in the
Micromeritics
instrument aforementioned. Powder X-ray diffraction (XRD) was performed with a Philips
PW1710 diffractometer, working with the Cu Kα line (λ = 0.154 nm). The
unit cell chemical
composition of all samples was determined by ICP-MS, using an octapole HP-7500c.
Results and discussion
Physico-chemical properties of both, parent and treated zeolites are shown in Table 2.
The treatment with CsOH results in a percentage of cesium between 18 and 19 %, whereas
the modification with Cs2CO3 obtains a cesium load of 16-18 %. As it could be expected, the
treatment with alkaline solutions leads to a displacement of ammonia desorption peak to
lower temperatures. Nitrogen physisorption data reveal a decrease in the microporous
volume and the surface area after the treatment.
Table 2. Chemical composition and morphological properties of the zeolites studied
According to the experimental procedure, during the TPD stage, the carbon dioxide
interacting directly with the adsorption sites is desorbed by the increase of the temperature.
Table 3 compares the results obtained from the CO2–TPD curves with
Cs-zeolites, carrying
out the adsorption at 50 ºC.
This desorption takes place in reverse order of the strength of the adsorption sites and
the adsorbate-adsorbent affinity. The desorbed amount of carbon dioxide detected in TPD
experiment is a function of the number of adsorption sites available in the adsorbent surface
(evaluated as mg CO2/g adsorbent), whereas the temperature of the peak can be considered
as a relative measurement of the strength of the adsorbate-adsorbent interactions.
Depending on the adsorbent, one or several peaks can be observed in the TPD curves. For
NaX zeolite only one desorption peak was detected, which indicated that there is only one