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ADSORPTION OF CARBON DIOXIDE ON ALKALI METAL EXCHANGED

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ADSORPTION OF CARBON DIOXIDE ON ALKALI METAL EXCHANGED

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 CO2free 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 CO2TPD 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

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