REGENERABLE SORBENT FOR CARBON DIOXIDE REMOVAL
FIELD OF THE INVENTION
The invention relates to a regenerable solid sorbent material suitable for C0 2 capture from a
gaseous stream, particularly exhaust gas streams characterized by relatively high temperatures and
relatively low C0 2 partial pressures, as well as processes and systems using such a sorbent and
methods of making such a sorbent.
BACKGROUND OF THE INVENTION
Combustion of fossil fuels is reported to be a major cause of the increased concentration of
carbon dioxide (C0 ) in the atmosphere. Although research is ongoing to improve energy
efficiency and to substitute low-carbon fuels to combat this problem, these methods will likely be
insufficient to limit the growth of atmospheric C0 2 concentrations to an acceptable level. As a
result, there is tremendous interest in the development of methods for preventing C0 2 release into
the atmosphere, i.e., carbon capture and storage (CCS) technology.
A number of technologies are available for removing C0 2 from a gaseous stream, including
wet chemical absorption with a solvent (e.g., using amines such as monoethanolamine or
diethanolamine), membrane separation, cryogenic fractionation, and adsorption using molecular
sieves. Another method for the removal of C0 2 from a gas stream involves dry scrubbing, meaning
treatment of the process gas with a dry, regenerable sorbent that removes C0 2 by chemical
absorption/ adsorption.
Existing technologies for C0 2 capture from gaseous streams suffer from a number of
drawbacks. The Department of Energy has reported that existing C0 2 capture technologies are not
cost-effective when considered in the context of large power plants. The net electricity produced
from existing plants would be significantly reduced upon implementation of many of these C0 2
capture technologies, since a high percentage of the power generated by the plant would have to be
used to capture and compress the C0 . Additionally, the process conditions under which the C0 2
must be removed in many applications render the existing technologies unusable. For example,
exhaust gas streams including automotive exhaust, cement kiln flue gas, steel mill flue gas, diesel
generator exhaust, and many other industrial and process gas streams are simply too hot (up to
600°C) for conventional post-combustion C0 capture technologies. Still further, the C0 2 partial
pressure of these gas streams is too low, typically less than 14.7 psia C0 2, for natural gas
sweetening or syngas C0 2 capture technologies to be effective. The combination of high
temperatures and low C0 2 partial pressures makes the development of a material capable of
effectively removing C0 2 from these gas streams a significant challenge.
US Pat. Nos. 5,480,625 and 5,681,503 are directed to sorbents for removing carbon dioxide
from habitable enclosed spaces, the sorbents including a metal oxide (e.g., silver oxide) as the
active agent and an alkali metal carbonate. However, the only exemplified sorbent regeneration
temperature range given is 160-220°C, too low to be useful for most exhaust gas applications.
US Pat. No. 6,280,503 describes a solid sorbent comprising magnesium oxide, preferably
promoted with an alkali metal carbonate or alkali metal bicarbonate, for removal of C0 2 from gas
streams at temperatures in the range of 300 to 500°C.
US Pat. No. 6,387,337 describes a C0 2 capture system that utilizes a sorbent in the form of
an alkali metal compound or an alkaline-earth metal compound, and that purportedly operates over
a temperature range of 200 to 2000°F.
US Pat. No. 6,387,845 is directed to a C0 2-absorbing sorbent comprising lithium silicate
optionally promoted by addition of an alkali metal carbonate, and which is capable of operation at
temperatures exceeding about 500°C.
US Pat. No. 7,314,847 is directed to a regenerable sorbent for C0 2 capture that includes a
binder in combination with one or more active components selected from alkali metal oxide, alkali
metal hydroxide, alkaline earth metal oxide, alkaline earth metal hydroxide, alkali titanate, alkali
zirconate, and alkali silicate. The sorbents are described as capable of operation over a temperature
range of25 o 600°C.
US Pat. No. 8,1 10,523 describes a sorbent for C0 2 capture that comprises an alkali metal
carbonate or bicarbonate combined with a high surface area support and a binder. The patent
suggests that the sorbent can operate over a temperature range of 40-200°C.
There is a continuing need in the art for the development of a sorbent material that is
capable of effectively removing C0 2 from gaseous streams, particularly exhaust gas streams
characterized by relatively high temperatures and relatively low C0 partial pressures.
SUMMARY OF THE INVENTION
The present invention provides a mixed salt composition adapted for use as a sorbent for
carbon dioxide removal from a gaseous stream, the composition being in solid form and comprising
i) magnesium oxide; ii) an alkali metal carbonate; and iii) an alkali metal nitrate, wherein the
composition has a molar excess of magnesium characterized by a Mg:X atomic ratio of at least
about 1.1:1, wherein X is the alkali metal. The sorbent is suitable for removing carbon dioxide
from a wide range of C0 -containing gaseous streams, and is particularly well-suited for C0 2
scrubbing of exhaust gases characterized by low C0 2 partial pressure and moderately high
temperature. In one embodiment, a pellet of the sorbent of the invention has a crush strength, as
determined by the Shell method, of at least about 0.3 MPa.
In certain embodiments, the alkali metal comprises sodium. The Mg:X atomic ratio can
vary, but will often be at least about 4:1 or at least about 6:1. The alkali metal nitrate is typically
present in an amount of at least about 1% by weight, based on total dry weight of the mixed salt
composition. An exemplary sorbent mixture is MgO:Na2C0 3:NaN0 3.
The invention also provides a method for removing carbon dioxide from a gaseous stream,
comprising contacting a gaseous stream containing carbon dioxide with a sorbent material
comprising the mixed salt composition of the invention. In certain embodiments, the contacting
step occurs at a temperature of about 100°C to about 450°C (e.g., about 250°C to about 375°C) and
a carbon dioxide partial pressure in the gaseous stream of about 1 to about 300 psia. In one
embodiment, the gaseous stream has a low carbon dioxide partial pressure, such as less than about
20 psia (e.g., less than about 14 psia, less than about 10 psia, less than about 5 psia, or less than
about 3 psia). In one embodiment, the sorbent material exhibits a C0 loading of at least about 10
weight percent C0 2 at an absorption temperature of about 250 to about 350°C.
The type of absorber housing the sorbent of the invention is not particularly limited, and can
include, for example, either a fixed bed or fluidized bed absorber. The sorbent is regenerable,
meaning the sorbent can be treated to cause desorption of carbon dioxide and reused. The
regenerating step can vary, with exemplary regeneration processes including pressure-swing
absorption, temperature-swing absorption, or a combination thereof. The regenerating step will
often include raising the temperature of the sorbent material or lowering the pressure applied to the
sorbent material, as compared to the temperature and pressure during the contacting step. In the
case of pressure swing absorption, the total pressure may remain constant or near constant while the
partial pressure of C0 2 is lowered, such as by purging with steam.
In a further aspect, the invention provides a process for preparing the mixed salt
composition of the invention, the process comprising: mixing a magnesium salt with a solution
containing alkali metal ions, carbonate ions, and optionally nitrate ions, to form a slurry or colloid
comprising a solid mixed salt comprising magnesium carbonate; separating the solid mixed salt
from the slurry or colloid to form a wet cake of the solid mixed salt; drying the wet cake to form a
dry cake comprising the solid mixed salt; and calcining the dry cake to form a mixed salt
composition according to the invention.
The mixing step can comprise mixing a first solution containing a dissolved alkali metal
carbonate, and optionally a dissolved alkali metal nitrate, and a second solution containing a
dissolved magnesium salt to form the solid mixed salt as a co-precipitate. In advantageous
embodiments, the magnesium salt used in the co-precipitation process is highly water soluble, such
as magnesium salts having a water solubility of at least about 10 g per 100 ml at 25°C and one
atmosphere, or at least about 40 g per 100 ml. Exemplary water soluble magnesium salts include
magnesium nitrate, magnesium acetate, and magnesium chloride. The carbonate ions in the solution
are typically derived from a precipitating salt comprising a carbonate ion added to the solution
(e.g., an alkali metal carbonate or ammonium carbonate).
Alternatively, the mixing step can include combining a solid magnesium salt (e.g., basic
magnesium carbonate) with the solution to form the solid mixed salt as a colloid. In one
embodiment, a solid magnesium salt is combined with a solution containing a dissolved alkali
metal carbonate and a dissolved alkali metal nitrate to form the solid mixed salt as a colloid.
BRIEF DESCRIPTION OF THE DRAWINGS
Having thus described the invention in general terms, reference will now be made to the
accompanying drawings, which are not necessarily drawn to scale, and wherein:
FIG. 1 is a flow chart illustrating a process for using the sorbent of the invention to remove
C0 2 from a gas stream;
FIG. 2 is an x-ray powder diffraction (XRD) pattern of a prepared sorbent according to the
invention showing the desired MgO:Na C0 3:NAN0 3 composition;
FIG. 3 graphically illustrates the C0 loading characteristics at different temperatures for
the sorbent prepared in Example 1 using a low C0 partial pressure feed gas;
FIG. 4 graphically illustrates the C0 2 loading characteristics at different temperatures for
the sorbent prepared in Example 2 using a low C0 2 partial pressure feed gas;
FIG. 5 graphically illustrates the C0 2 loading characteristics at two temperatures for the
sorbent prepared in Example 1 using a feed gas of varying C0 2 partial pressure;
FIG. 6 graphically illustrates the effect of alkali metal selection on C0 2 loading
characteristics of a sorbent according to the invention;
FIG. 7 graphically illustrates the effect of magnesium salt selection on C0 loading
characteristics of a sorbent according to the invention;
FIG. 8 graphically illustrates the effect of Mg:Na atomic ratio in the reagent mixture on C0 2
loading characteristics of a sorbent according to the invention;
FIG. 9 graphically illustrates the effect of precipitation solution concentration on C0
loading characteristics of a sorbent according to the invention;
FIG. 10 graphically illustrates the effect of precipitating agent selection on C0 loading
characteristics of a sorbent according to the invention;
FIG. 1 1 graphically illustrates the effect of production method on C0 2 loading
characteristics of a sorbent according to the invention;
FIG. 12 graphically illustrates the effect of magnesium oxide source in a gelation
production method on C0 2 loading characteristics of a sorbent according to the invention; and
FIG. 13 graphically illustrates the effect of drying process selection on C0 loading
characteristics of a sorbent according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention now will be described more fully hereinafter with reference to the
accompanying drawings, in which some, but not all embodiments of the inventions are shown.
Indeed, these inventions may be embodied in many different forms and should not be construed as
limited to the embodiments set forth herein; rather, these embodiments are provided so that this
disclosure will satisfy applicable legal requirements. Like numbers refer to like elements. As used
in the specification, and in the appended claims, the singular forms "a", "an" , "the", include plural
referents unless the context clearly dictates otherwise.
The present invention provides a mixed salt composition adapted for use as a sorbent for
carbon dioxide removal from a gaseous stream, the composition being in solid form and comprising
magnesium oxide; an alkali metal carbonate; and an alkali metal nitrate, wherein the composition
has a molar excess of magnesium characterized by a Mg:X atomic ratio of at least about 1.1:1,
wherein X is the alkali metal. Exemplary ranges of Mg:X atomic ratio include at least about 2:1, at
least about 3:1, at least about 4:1, at least about 5:1, at least about 6:1, at least about 7:1, and at
least about 8:1. Note that the two alkali metal salts in the sorbent will typically comprise the same
alkali metal (e.g., sodium), although mixtures of different alkali metals could be used without
departing from the invention. One embodiment of the sorbent of the invention is the mixture
MgO:Na C0 3:NaN0 3. Although not bound by a theory of operation, it is believed that the sorbent
of the invention loads C0 2 in the form of magnesium carbonate (MgC0 3), while the alkali metal
component of the sorbent promotes the reactions through which C0 is captured by the sorbent.
Other acid gas components of a process gas stream can also be removed using the sorbent of the
invention, such as H S, COS, S0 , NOc species, and the like. In some cases, these acid gases may
be irreversibly absorbed.
The relative amount of Mg and alkali metal can be characterized in terms of mass. In
certain embodiments, the mixed salt sorbent comprises at least about 60% by weight magnesium
oxide (based on total dry weight of the mixed salt composition), more often at least about 70%, at
least about 75%, or at least about 80% by weight magnesium oxide (e.g., a MgO weight range of
about 70% to about 90%>). In certain embodiments, the mixed salt sorbent comprises at least about
8% by weight of alkali metal carbonate (e.g., sodium carbonate), based on total dry weight of the
mixed salt composition, such as at least about 10% by weight, at least about 12% by weight, or at
least about 14% by weight (e.g., an alkali metal carbonate weight range of about 8% to about 18%).
In certain embodiments, the mixed salt sorbent comprises at least about 1% by weight of alkali
metal nitrate (e.g., sodium nitrate), based on total dry weight of the mixed salt composition, such as
at least about 2% by weight, at least about 3% by weight, or at least about 5% by weight (e.g., an
alkali metal nitrate weight range of about 1% to about 10%).
As used herein, "magnesium salt" refers to an ionic compound comprising magnesium as a
cation. "Alkali metal salt" refers to an ionic compound comprising an alkali metal as the cation.
The alkali metal (i.e., a Group 1 element, formerly known as Group IA elements) can vary, and
expressly includes lithium, sodium, potassium, rubidium, caesium, and francium. The anions
associated with either the magnesium salts or alkali metal salts can vary, with specific examples
including carbonate, acetate, chloride, hydroxide, oxide, and nitrate groups.
The mixed salt sorbent of the invention can be characterized by C0 2 loading ability.
Certain embodiments of the sorbent of the invention are capable of achieving C0 2 loading of at
least about 10 weight percent C0 2, at least about 15 weight percent C0 2, at least about 20 weight
percent C0 2, or at least about 25 weight percent C0 (e.g. about 10 to about 30 weight percent
C0 2). Although the composition of the sorbent can impact peak absorption temperature, in one
embodiment, the above-noted C0 2 loading levels are achieved at an absorption temperature in the
range of about 250 to about 350°C.
The sorbent composition is typically used in a particulate form with a particle size of about
150-425 mesh, although other particle size ranges could be used without departing from the
invention. In certain embodiments, the sorbent composition can include additional components.
For example the sorbent can include one or more inert binders for various purposes, such as to
facilitate granulation or extrusion, improve handling, enhance particle strength, or reduce pressure
drop across packed sorbent beds. Exemplary binders include alkali silicates, inorganic clays,
boehmite, and organic binders (e.g., starch, cellulosic polymers such as methylcellulose, polyvinyl
acetate, and lignin sulfonate). Binders used in the present invention should be chemically stable at
the operating temperature of the sorbent (e.g., about 100 to about 450°C). In some cases, certain
binders are detrimental to the performance of the sorbent. For example, it has been determined that
the use of boehmite is generally disadvantageous because the presence of boehmite reduces C0 2
loading performance, presumably due to interaction between the alkali metal promoter of the
sorbent and the boehmite.
Certain binders, such as methylcellulose, can impart porosity to the final sorbent
composition. Organic binders of this type are removed during the calcinations step, leaving a pore
network within the sorbent extrudate.
Although some binder materials can also enhance particle strength of sorbents, it has been
determined that the sorbent of the invention may not require a binder to enhance particle strength
because the crush strength of the sorbent of the invention made without binder was found to be
approximately equivalent to a sorbent composition of the invention that included a boehmite
binder. The crush strength of a pellet (i.e., extrudate) of the sorbent of the invention, as determined
by the Shell method, is typically at least about 0.3 MPa, at least about 0.4 MPa, or at least about 0.5
MPa (e.g., a crush strength range of about 0.3 MPa to about 1 MPa).
The sorbent composition of the invention can further include a porous material as a carrier
for the mixed salt composition. Exemplary porous carriers include bauxite, activated carbon, clays
(e.g., amorphous, crystalline, or mixed layer clays), metal oxides (e.g., iron oxide, alumina,
magnesium oxide, and zirconium oxide), magnesium silicate, molecular sieves, silica gel, and
zeolites.
Although the sorbent of the invention has a large molar excess of Mg over alkali metal, the
initial reagent mixture used to produce the final product will typically exhibit a molar excess of
alkali metal. As the alkali metal is more soluble in water, more of the alkali metal will remain in
solution during the reactions that lead to formation of the final mixed salt composition, and more
alkali metal is lost during steps taken to effect separation of the solid mixed salt from the solution
(e.g., filtration or washing steps).
As shown in Example 9, the extent of the molar excess of the alkali metal over the
magnesium in the initial reagent mixture used in the co-precipitation method can vary, and will
impact the level of C0 2 loading and optimal absorption temperature for the sorbent. Greater alkali
metal content in terms of Mg:X atomic ratio, such as greater than about 1:3, greater than about 1:4,
greater than about 1:5, greater than about 1:6, greater than about 1:7, or greater than about 1:8, tend
to increase both the maximum C0 2 loading capacity of the sorbent as well as the optimal
absorption temperature. Lower molar excesses may be appropriate where a lower absorption
temperature is desirable. A typical molar excess range for the co-precipitation method is an Mg:X
ratio of about 1:3 to about 1:10 for the initial reagent mixture.
The mixed salt sorbent compositions of the invention can be made by any process that
facilitates formation of a solid mixed salt from solution (e.g., co-precipitation or gelation/colloid
processes). The salts used in the process are chosen such that, upon reaction, MgC0 3 is formed in
the precipitate. The process of the invention will typically involve mixing a magnesium salt and an
alkali metal salt in the presence of carbonate and nitrate ions in solution. The carbonate ions are
typically provided by a precipitating agent in order to drive formation of the desired magnesium
salt. The precipitating agent and the alkali metal salt can be the same reagent in some
embodiments, meaning the alkali metal salt is a carbonate salt, thereby providing both the
necessary alkali metal content and carbonate ions. In a typical co-precipitation process, both the
magnesium salt and the alkali metal salts are provided in a dissolved form and the solutions of each
salt are combined and mixed, which produces the desired precipitate of the mixed salt sorbent of
the invention. The mixing step typically comprises mixing a first solution containing dissolved
alkali metal carbonate and nitrate salts (e.g., sodium carbonate and sodium nitrate) and a second
solution containing a dissolved magnesium salt (e.g., magnesium nitrate) to form the solid mixed
salt of the invention as a co-precipitate.
The carbonate ions in the solution are derived from a precipitating salt comprising a
carbonate ion added to the solution. As noted above, the alkali metal salt itself can be the source of
carbonate ions (e.g., sodium carbonate), or other sources of carbonate ions can be used in addition
to the alkali metal salt (e.g., ammonium carbonate). At least one of the salts mixed in the mixing
step is a nitrate salt and at least one of the salts mixed in the mixing step is a carbonate salt. In
addition to reagent-grade alkali metal salts that are commercially available, commercial-grade
reagents can also be used in some cases, such as the use of soda ash as a sodium carbonate source.
As noted in Example 8 below, the water solubility of the magnesium salt used in the coprecipitation
process can have a significant impact on the performance of the sorbent, both in terms
of maximum C0 2 loading and optimal absorption temperature. The use of magnesium salts with
higher water solubility levels enhance C0 2 loading. In certain embodiments, the water solubility of
the magnesium salt is at least about 10 g per 100 ml (at 25°C and one atmosphere), or at least about
40 g per 100 ml. Exemplary water soluble magnesium salts include magnesium nitrate, magnesium
acetate, and magnesium chloride.
Co-precipitation processes where each salt is initially in solution can be difficult to scale up
because of the large quantity of solvent required. Accordingly, in certain embodiments, the mixed
salt sorbent is formed by a process where at least one of the salt components (typically the
magnesium salt) is added in solid form to a solution of the other salts (typically the alkali metal
salts), resulting in colloidal gelation of the desired mixed salt sorbent of the invention. Such a
process forms a stable colloid of the desired solid mixed salt dispersed in the solution. It has been
determined that the gelation process described herein can be used to scale up production (e.g., to
300 g batch size) of the sorbent of the invention without losing carbon dioxide capture
performance.
In one embodiment of the gelation process, a solid magnesium salt such as basic
magnesium carbonate (e.g., magnesium carbonate hydroxide having the formula
4MgC0 3 Mg(OH)2 x H20), which is essentially water-insoluble, is mixed with a solution
containing alkali metal ions, carbonate ions, and nitrate ions. The solution will typically comprise a
dissolved alkali metal carbonate (and/or other carbonate ion sources) and a dissolved alkali metal
nitrate. Mixing the solid magnesium salt with the dissolved alkali metal solution results in liquidsolid
reactions, and ultimately, formation of a stable colloid of the desired mixed salt sorbent
material dispersed in the solution. In the gelation process, the formation of the desired mixed salt
product proceeds in a manner similar to a sol-gel process, with the mixed salt product forming as a
dispersed, stable colloidal phase or network within the solution. It is noted that the manner in
which the solid magnesium salt is contacted with the solution of alkali metal ions, carbonate ions,
and nitrate ions can vary. Powders of the magnesium salt and various alkali metal salts (e.g.,
sodium carbonate and sodium nitrate) can be premixed in dry form using milling/grinding
techniques known in the art and then combined with water to create the desired solution.
Alternatively, the solution of alkali metal ions, carbonate ions, and nitrate ions can be prepared
first, followed by addition of the solid magnesium salt.
In either a colloid or co-precipitation process, the process steps typically involve:
i) mixing a magnesium salt with a solution containing alkali metal ions, carbonate
ions, and nitrate ions to form a slurry or colloid comprising a solid mixed salt,
wherein the mixture of the alkali metal salt with the magnesium salt in solution has a
molar excess of alkali metal characterized by a Mg:X atomic ratio of at least about
1:3, wherein X is the alkali metal;
ii) separating the solid mixed salt from the slurry or colloid to form a wet cake of the
solid mixed salt;
iii) drying the wet cake to form a dry cake comprising the solid mixed salt; and
iv) calcining the dry cake to form a mixed salt sorbent composition according to the
invention.
The separating and drying steps can be conducted using any conventional separation and
drying equipment and techniques known in the art. Typical separating steps include centrifugation
and/or filtration, optionally accompanied by one or more washing steps. Exemplary drying
equipment includes spray dryers, rotary dryers, flash dryers, conveyer dryers, fluid bed dryers, and
the like. The temperature and time of the drying step can vary, depending on the desired moisture
content of the dried cake. A typical temperature range is about 50°C to about 150°C and a typical
drying time is about 2 to about 24 hours.
Following drying, the dried cake is preferably subjected to a calcining step. This step
enhances the C0 2 loading capacity of the sorbent. Calcining temperatures and times can vary
depending on the desired final characteristics of the sorbent, but will typically involve a maximum
temperature in the range of 400 to 500°C and a time of about 2 to about 10 hours. The calcining is
conducted by ramping up the temperature of the sorbent at a ramp rate of, for example, about 1 to
about 5°C per minute. Following calcining, the mixed salt composition will be in dry powder form.
The particle size of the powder can be adjusted as desired using milling or grinding equipment
known in the art.
The sorbent powder can also be optionally combined with a binder and extruded before
final processing into the desired granule size. The extruding step can also occur before calcination
so that the calcinations process drives off any organic binder material present in the final sorbent
extrudate. Still further, the sorbent powder can be admixed with a porous carrier using known
techniques in the art, such as by mixing the porous carrier with a slurry of the sorbent powder
followed by drying of the treated carrier. In addition, the sorbent material can be slurried and
spray-dried to form fluidizable particles.
If a sorbent extrudate is formed, the drying procedure used for the extrudate can impact the
crush strength of the extrudate pellet. Crush strength declines with increases in drying rate.
Accordingly, allowing the extrudate to remain at room temperature before subjecting the sorbent to
higher temperatures is advantageous. Further, lower ramp rates during drying are useful to reduce
the impact of drying on crush strength, such as temperature ramp rates of less than about
0.5°C/min, or less than about 0.4°C/min, or less than about 0.3°C/min.
The mixed salt sorbent composition of the invention can be used to remove carbon dioxide
from a gaseous stream by contacting a gaseous stream containing carbon dioxide with a sorbent
material comprising the mixed salt composition of the invention for a time and at a temperature
sufficient for the sorbent to remove all or a portion of the C0 from the process gas stream. The
process of using the sorbent is set forth schematically in Fig. 1. A process gas containing C0 2 is
received in step 10 and contacted with the sorbent of the invention in step 20. A treated process gas
having a reduced C0 2 content can be withdrawn in step 30. The sorbent will eventually become
saturated with C0 2 and require regeneration as noted in step 40, which usually involves passing an
inert gas through the sorbent and changing the temperature or pressure conditions of the sorbent to
facilitate desorption of C0 2 from the sorbent. The regeneration step may result in production of a
concentrated C0 2 gaseous stream in step 50. However, if a high quality C0 2 product gas is not
desired, a highly diluted C0 2 gaseous stream may be produced by purging with a hot gas such as
steam or air or another diluent.
The process gas to be treated according to the invention can vary. Any gaseous mixture
comprising C0 2 where it is desired to reduce the C0 2 concentration of the gaseous mixture would
be suitable for use in the present invention. Exemplary process gases include any exhaust gas from
a fossil fuel combustion process (e.g., exhaust flue gas streams produced by fossil fuel-fired power
plants including industrial boilers, exhaust from vehicles with internal combustion engines, cement
kiln flue gas, steel mill flue gas, glass manufacturing flue gas, and diesel generator exhaust) or a
syngas produced by the gasification of coal or reforming of natural gases. The sorbent of the
invention could be used, for example, in advanced power systems such as Integrated Gasification
Combined Cycle (IGCC), Low-Emissions Boiler Systems (LEBS), High Performance Power
Systems (HIPPS), and Pressurized Fluid Bed Combustors (PFB).
Of particular interest are chemical conversion processes in which C0 2 is an undesirable by¬
product or a contaminant, such as the conversion of warm syngas to hydrogen, such as in the
context of hydrogen production from syngas derived from coal, biomass, or natural gas for power
generation (for example in a gas turbine); hydrogen production for chemical conversions such as
ammonia; or syngas production with desired ratio of H2-to-CO for production of methanol and
Fischer Tropsch products. Incorporating the sorbent of the invention into a syngas treatment
process could consist of stand-alone C0 2 removal from syngas, water-gas shift of syngas followed
by C0 2 removal using the sorbent of the invention, or simultaneous water-gas shift of syngas and
C0 2 removal known as sorption-enhanced water-gas shift. Additional uses of the sorbent of the
invention could include use in the process of direct conversion (i.e., reforming and partial
oxidation) of carbonaceous fuels to hydrogen and syngas, or C0 2 scrubbing of recycle streams in
chemical conversion processes, such as recycle streams involved in ethylene oxide production,
oxidative coupling of methane and ethane, or dimethyl ether production.
The partial pressure of C0 2 in the process gas to be treated with the sorbent of the invention
can vary. A typical C0 2 partial pressure range for the process gas is about 1 to about 300 psia. In
one embodiment, as exemplified in Example 5, the sorbent is effective at removing C0 2 from
gaseous streams characterized by high temperature (e.g., greater than about 400°C or greater than
about 425°C) and high partial pressure of C0 2 (e.g., greater than about 30 psia, greater than about
50, or greater than about 80 psia). However, the sorbent of the invention is also effective at process
conditions associated with many types of exhaust gases; namely, moderately high temperatures
(e.g., about 100 to about 450°C, more typically about 250°C to about 375°C) and low C0 2 partial
pressure (e.g., less than about 20 psia, less than about 14 psia, less than about 10 psia, or less than
about 5 psia, or less than about 3 psia).
The manner in which the sorbent material is contacted with the process gas can vary.
Typically, the sorbent is housed within an absorber in a bed and the process gas passes through the
bed. The bed of sorbent can be in a fixed bed or fluidized bed configuration using absorber and
regenerator designs known in the art. If a fixed bed of sorbent is used, the system may include
multiple sorbent beds in parallel arrangement such that beds in need of regeneration can be taken
offline and regenerated. In this embodiment, the same vessel is used as both an absorber and a
regenerator by simply changing the gas flowing through the vessel as well as the temperature
and/or pressure in the vessel. In another embodiment, a fluidized bed of sorbent is used, and a
separate absorber and regenerator in fluid communication can be used. In this embodiment, C0 2
loaded sorbent travels from the absorber to a separate regenerator vessel where C0 2 is stripped
from the sorbent before the sorbent is transported back to the absorber in a continuous or semicontinuous
flow.
The method used to regenerate the sorbent will vary, but will usually involve changing the
temperature or pressure experienced by the bed of sorbent to facilitate release/desorption of the
C0 2 bound in the mixed salt composition. Known methods of regenerating sorbents can be used,
such as pressure-swing absorption (PSA), including vacuum swing absorption, temperature-swing
absorption (TSA), or a combination thereof (e.g., combined TSA-PSA processes). Such
regenerating processes involve one or more of raising the temperature or lowering the pressure
applied to the sorbent to desorb C0 2 into an inert gas (e.g., nitrogen or steam) passing through the
sorbent bed. In certain embodiments, the sorbent of the invention is capable of regeneration at a
temperature of about 375 to about 450°C
The sorbent will eventually become saturated with C0 2, and the level of C0 2 loading in the
sorbent material can be determined by measuring and comparing the content of C0 in the process
gas stream before and after contact with the sorbent. When it is evident that no further C0 2 is
being removed from the process gas stream, the sorbent can be regenerated by, e.g., heating it to the
desorption temperature. By measuring the amount of C0 2 contained in the concentrated C0 2 gas
stream exiting the regenerating sorbent, the skilled artisan can determine when the sorbent is ready
for reuse. The C0 gaseous stream produced by sorbent regeneration can be sequestered as known
in the art or used as a raw material in processes requiring C0 2, such as in production of various
chemicals; as a component of fire extinguishing systems; for carbonation of soft drinks; for
freezing of food products; for enhancement of oil recovery from oil wells; and for treatment of
alkaline water.
EXAMPLES
Example 1: Sorbent Prepared by Gelation
A sorbent comprising MgO:Na2C0 3:NaN0 3 at a mass ratio of 75.8:16:8.2 was prepared as
follows. An amount (395 g) of magnesium carbonate hydroxide (4MgC0 3 · Mg(OH)2 x H20 ) was
added to 800 ml of a solution of sodium carbonate (42.18 g) and sodium nitrate (21.63 g) dissolved
in deionized water. The resulting mixed salt colloid was stirred for 30 minutes, covered, and
allowed to sit overnight (up to 16 hours) at ambient temperature. Thereafter, the colloid was dried
in an oven at 120°C overnight (up to 16 hours) to form a dry cake.
The dry cake was then calcined by heating from 120°C to 450°C, at a ramp rate of
3°C/minute, followed by holding at a temperature of 450°C for 4 hours. The calcined cake was
crushed and sieved to collect a 150-425 mesh fraction.
Example 2 : Sorbent Prepared by Co-Precipitation
A magnesium-sodium mixed salt sorbent was prepared by precipitating a solid from two
starting solutions. A first solution containing 233.4 g of Na2C 3 dissolved in 3000 ml deionized
water was placed in a 5.0 liter plastic beaker, and stirred vigorously with a mechanical agitator. A
second solution of 188.4 g Mg(N0 3)2 : 6 H20 in 500 ml of deionized water was pumped into the
first solution at a rate of approximately 30 ml/minute. The resulting slurry was stirred for an hour
and then covered and stored overnight under ambient conditions. Thereafter, the slurry was filtered
using a vacuum-assisted Buchner funnel assembly to collect a wet precipitate cake. About 3200 ml
of filtrate was collected and then dried in an oven at 120°C for 24 hours to form a dry cake. The
dried cake was then calcined, crushed and sieved as described in Example 1. A combination of
inductively coupled plasma (ICP) analysis and elemental analysis (CHONS) was used to
determine/estimate the molecular species composition of the calcined material. The sorbent was
found to have an approximate mass composition of MgO:Na2C0 3:NaN0 3 of 86.8:8.8:4.4 and a
Mg:Na molar ratio of 9.8: 1. An XRD pattern was collected for the calcined sorbent powder and the
presence of MgO, Na2C0 3, and NaN0 3 was clearly observed, as shown in Fig. 2, verifying that the
prepared sorbent had the desired MgO:Na2C0 3:NaN0 3.
Example 3 : Carbon Dioxide Loading of Sorbent of Example 1
The amount of C0 loaded on the sorbent of Example 1 was evaluated using a simulated
exhaust gas consisting of 13% C0 2, 13% H20 , and balance N2 (i.e., an exhaust gas with a C0 2
partial pressure of 1.9 psia) using a conventional, packed-bed reactor system equipped with a
Horiba NDIR C0 2 analyzer to measure the concentration of C0 in the gas entering and exiting the
reactor. The packed-bed reactor was loaded with 6 g of the prepared sorbent of Example 1 and a
quantity of an inert, silicon carbide (SiC), to occupy the additional reactor volume. The reactor was
then heated to 450°C at 10 °C/min in flowing N2 to activate the sorbent and was held at this
temperature until the C0 2 concentration in the reactor effluent dropped below 0.1%. The reactor
was cooled to the lowest absorption temperature in flowing N . Once the reactor stabilized at the
desired absorption temperature, the composition of the simulated feed gas (13% C0 2, 13%» H20 ,
bal. N ) was verified by the C0 2 analyzer. When the C0 concentration was stable, +/- 0.1% from
set point, for a minimum of 5 minutes, the simulated exhaust gas was fed to the reactor feed. The
C0 concentration of the reactor effluent was continuously measured by the C0 2 analyzer and the
absorption phase of the cycle was continued until the C0 concentration in the effluent reached
90% of the previously measured feed concentration. This corresponds to a 90%> breakthrough. At
this point, the feed gas was changed to pure N2 and the temperature for the reactor was ramped at
5°C/min to 450°C. The reactor was maintained at 450°C until the C0 2 concentration in the reactor
effluent decreased below 0.1 vol%, or a period of 2 hours was exceeded, indicating the completion
of sorbent regeneration. The reactor temperature was then reduced to the desired absorption
temperature, and the absorption-regeneration procedure described above was repeated.
Fig. 3 indicates the amount of carbon dioxide loaded on the sorbent over a range of
temperatures from 200 to 425°C in 25°C increments. As shown, the test illustrates the
effectiveness of the sorbent at absorbing C0 over a wide temperature range with a maximum
loading at approximately 300°C.
Example 4 ; Carbon Dioxide Loading of Sorbent of Example 2
The C0 loading ability of the sorbent of Example 2 was analyzed using the same
experimental process outlined in Example 3. Fig. 4 indicates the amount of carbon dioxide loaded
on the sorbent of Example 2 over a range of temperatures from 100 to 425°C. As shown, the test
illustrates the effectiveness of the sorbent at absorbing C0 over a wide temperature range with a
maximum loading at approximately 350°C.
Example 5: Carbon Dioxide Loading of Sorbent of Example 1 Using High CO? Partial Pressure
Process Gas
The mixed salt sorbent described Example 1 was evaluated for removal of C0 from warm,
high C0 partial pressure process gas streams. There are numerous examples of industriallyrelevant
process gas streams that can be described as warm, high C0 partial pressure process gas
streams, such as desulfurized syngas, high-temperature and low-temperature shifted syngas, and
C0 -containing hydrogen.
In this example, simple gas mixtures containing C0 and N with various C0 2 partial
pressures were used to simulate warm, high C0 2 partial pressure process gas streams. C0 2 uptake
and release measurements were made using a conventional, packed-bed reactor equipped with a
Horiba NDIR C0 analyzer to measure the concentration of C0 2 in the gas entering and exiting the
reactor. The packed-bed reactor was loaded with 4 g of the prepared sorbent of Example 1 and a
quantity of an inert material (silicon carbide) was intermixed with the sorbent to occupy the
remaining reactor volume. The reactor was then heated to 450°C at 10 °C/min in approximately
100 ml/min of N2 to activate the sorbent and was held at this temperature until the C0 2
concentration in the reactor effluent dropped below 0.1%. The reactor pressure was then elevated
to and maintained at 300 psia by a pressure control valve located downstream of the reactor. The
reactor was cooled to the desired absorption temperature typically ranging between 375°C and
450°C. Once the reactor stabilized at the desired absorption temperature, the composition of the
simulated warm process gas, containing CO2 and N2, was verified by the CO2 analyzer positioned
downstream of the pressure control valve. When the CO2 concentration was stable, +/- 0.1% from
set point, for a minimum of 5 minutes, the simulated process gas was fed to the reactor.
The C0 2 partial pressures evaluated ranged from 15 psia to 150 psia. The CO
concentration of the reactor effluent was continuously measured by the CO2 analyzer and the
absorption phase of the cycle was continued until the C0 2 concentration in the effluent reached
90% of the previously measured feed concentration. This corresponds to a 90%» breakthrough. The
amount of C0 absorbed by the sorbent, the CO uptake, was determined by integration of the
difference between the mass flow rates of CO2 entering and exiting the reactor. Once 90%
breakthrough had been reached, the feed gas was changed to either pure N2 or a CO2 mixture
having lower CO2 content than used in the absorption stage. The reactor temperature was either
maintained at the absorption temperature or reduced to a lower temperature. The reactor remained
in the regeneration stage until the CO2 concentration in the reactor effluent decreased to < 0.1 vol%
greater than the CO2 concentration in the feed stream. The reactor temperature and CO2
concentration was then returned to the desired absorption conditions and the experiment could be
repeated.
Results provided in Fig. 5 report the CO2 loading capacity of the Mg-Na mixed salt sorbent
as a function of CO partial pressure and absorption temperature. Increasing the CO2 partial
pressure in the simulated warm process gas resulted in large increases in the CO2 loading. In this
study, the maximum CO2 partial pressure evaluated was 150 psia, corresponding to a 50-50 mixture
of C0 2 and N2 with a total pressure of 300 psia. At this C0 2 partial pressure, the sorbent was
capable of loading 52.2 wt% C0 2 and 49.6 wt% C0 at 430°C and 450°C respectively. For C0
partial pressures < 20 psia C0 , the sorbent did not absorb measureable quantities of C0 for either
temperature evaluated. The shape of the C0 2 loading curve, having a rapid decrease in C0 2
loading with decreasing C0 partial pressure below 100 psia, is very promising for temperatureswing,
pressure-swing, and partial-pressure swing absorption process arrangements and process
arrangements consisting of combinations of temperature and pressure swing. These results also
indicate that embodiments of the sorbent of the invention are useful for high temperature, high C0
partial pressure applications in addition to exhaust gas applications characterized by more moderate
temperatures and very low C0 2 partial pressures.
Example 6 : Regeneration of Sorbent of Example 1
The maximum C0 2 partial pressure that can be realized in the regeneration off-gas at a
prescribed temperature was determined. In these experiments, the sorbent of Example 1 was
loaded with C0 at 450°C and a C0 partial pressure of 150 psia using an experimental system
essentially as described in Example 5. The C0 -loaded sorbent was then regenerated by cooling
from 450°C to the desired regeneration temperature without flow and once the desired temperature
was reached, the feed gas was switched to 15 psia C0 2 balance N2. The C0 2 content of the gas
exiting the reactor was measured by a downstream NDIR C0 2 analyzer. Results presented in the
table below report the maximum C0 partial pressure (ppC0 ) observed during regeneration of the
sorbent at the indicated temperature in a regeneration gas having a C0 partial pressure of 15 psia.
These results indicate the maximum C0 2 partial pressure that can be realized in the regeneration
off-gas at the prescribed temperature. The maximum C0 partial pressure that can be realized
during sorbent regeneration was found to decrease from 450°C to 410°C. At 410°C, the sorbent
was found to not regenerate at all and therefore, the maximum C0 2 partial pressure in the
regeneration off-gas is less than 15 psia C0 2. These results indicate that the Mg-Na sorbent of
Example 1 can be regenerated by in a partial pressure swing process combined with a negative
temperature swing.
Table 1
Example 7 : Effect of Alkali Element on Sorbent Performance
The effect of the alkali element in the mixed salt sorbent was evaluated by preparing
sorbents with the first three alkali earth metals in the Periodic Table of the Elements, specifically:
Lithium (Li), Sodium (Na), and Potassium (K). The mixed salt sorbents were prepared following
the same co-precipitation preparation and having the same Mg:Alkali Metal molar ratio of 1:6. The
prepared sorbents were evaluated for the removal of C0 2 from simulated exhaust gas in a fixed-bed
reactor system at the experimental conditions (temperature, gas composition, and gas hourly space
velocity (GHSV)) provided in Table 2 below.
Table 2
Absorption
Temperature: 100 to 450°C
Gas Composition: 13% C0 2, 13% H20 , Bal. N2
GHSV: 3,125 h 1
Regeneration
Temperature : Ramp to 450°C at 10°C/min
Gas Composition: N2
GHSV: 2,500 h 1
The effect of the alkali element on the performance of the mixed salt sorbent is illustrated in
These results suggest that the selection of the alkali element (e.g., Li, Na, K) can be used to
tune the sorbent' s window of operation. From these results, it appears that sorbents containing
sodium (Na) provide the best operational temperature range for many applications, and such
sorbents are also capable of achieving the highest C0 2 loading. However, sorbents containing Li or
K were also shown to absorb carbon dioxide. The sorbent containing sodium absorbed C0 2 over a
temperature range of about 100°C to about 425°C, reaching a maximum at about 350°C. The
sorbent containing lithium was most effective at 200°C and showed absorption of C0 2 over a
temperature range of about 200°C to about 275°C, while the compound containing potassium
absorbed C0 2 at a higher temperature ranging from about 300°C to about 425°C with a peak at
about 350°C.
Example 8: Effect of Magnesium Source on Sorbent Performance (Co-Precipitation Method)
One of the preparation parameters that can affect the composition and performance of the
sorbent is the source of magnesium, which can affect the salt species formed during precipitation.
Since magnesium carbonate or magnesium oxide is the targeted magnesium compounds in the salt
mixture, selection of the magnesium source that preferentially leads to the formation of these
species is desired. In this study, the effect of the magnesium source of the performance of the
mixed salt sorbent was evaluated by preparing sorbents from magnesium nitrate (Mg(N0 3)2),
magnesium oxide (MgO), and magnesium hydroxide (Mg(OH) ) . These sorbents were prepared
following the same co-precipitation preparation procedure with a Mg:Na molar ratio of 1:6. The
prepared sorbents were evaluated for the removal of C0 2 from simulated exhaust gas in a fixed-bed
reactor system at the same experimental conditions (temperature, gas composition, and gas hourly
space velocity (GHSV)) used in Example 7.
The effect of the magnesium source on the C0 2 loading as a function of temperature for the
mixed salt sorbents is provided in Fig. 7. As can be seen, the sorbent prepared from magnesium
nitrate achieved significantly greater C0 loadings than the oxide or hydroxide sorbents, indicating
that the magnesium source has a significant impact on the performance of the sorbent. However,
all three tested magnesium salts produced a sorbent that absorbed carbon dioxide. The primary
difference between these magnesium sources is the solubility of the salt in water. For example, the
solubility of magnesium nitrate in water is 125 g /100 ml, whereas the solubility of magnesium
hydroxide is 1.2 mg /100 ml.
Although not bound by any particular theory of operation, although the same preparation
procedure was followed, it appears that the resulting sorbent materials were formed via different
pathways. The magnesium nitrate-prepared mixed salt was likely formed by the addition of a
solution of sodium carbonate (Na C0 3) to a solution containing completely dissolved magnesium
nitrate. Upon the addition of sodium carbonate, a white precipitate, the mixed salt, was formed.
Precipitation likely occurred due to anion exchange between the magnesium and sodium cations in
which a mixture of magnesium carbonate, hydroxide, and nitrate and sodium nitrate and carbonate
was formed.
The sorbents made using magnesium oxide and hydroxide likely followed a different
pathway due to their limited solubility in water. Following the same procedure, a solution
containing sodium carbonate was slowly added to a solution containing a well-mixed slurry of
magnesium hydroxide. Due to the presence of a precipitate, it was not possible to observe or
distinguish the precipitation of a mixed salt species.
The C0 2 loading results indicate that the magnesium source, and specifically the solubility
of the source compound in water, is a very important parameter in the preparation of a mixed salt
sorbent with high C0 2 loading capacity. Although sorbent prepared using magnesium nitrate
exhibited very good C0 2 loading capacity, other highly water soluble magnesium salts, such as
magnesium chloride (54.3 g / 100 ml) and magnesium acetate (39.6 g / 100 ml), would also be
useful for producing sorbents of the invention.
Example 9 : Effect of Mg:Na Molar Ratio on Sorbent Performance
It is understood that C0 2 is loaded on the sorbent of the invention in the form of MgC0 3,
which has been verified by XRD, and that the sodium species, although clearly involved in the C0 2
capture mechanism, do not store C0 2. Therefore, to more thoroughly understand the role of
sodium in the C0 2 capture mechanism in the mixed salt sorbent, several sorbent samples were
prepared with Mg:Na molar ratios in the reagent mixture ranging from 1:3 to 1:8. The sorbents
were prepared following the same co-precipitation preparation procedure with the exception of the
quantity of Na2C0 3 used during the precipitation stage. The prepared sorbents were evaluated for
the removal of C0 2 from simulated exhaust gas in the fixed-bed reactor system at the same
experimental conditions (temperature, gas composition, and gas hourly space velocity (GHSV))
used in Example 7.
The experimental results presented in Fig. 8 indicate that the Mg:Na molar ratio does affect
the C0 2 loading of the sorbent. For sorbents preparing using molar excesses of sodium (e.g., 1:6
and 1:8), the performance of the sorbent is consistent with previous findings. The C0 2 loading
capacity increases with absorption temperature, passes through a maximum of approximately 13
wt% C0 2 at 350°C, and rapidly decreases with increasing absorption temperature. Increasing the
sodium molar excess from 6 to 8 appears to have little effect on the general shape of the absorption
curve. The peak C0 2 loading is approximately 12 wt% at 350°C for both materials, and both
exhibit rapid decrease in C0 2 loading with increasing temperature. The only observable difference
is a slight decrease in the C0 loading for temperatures below 350°C for the sorbent with a larger
quantity of sodium.
Decreasing the sodium content to 1:4 and below (e.g., 1:3) appears to significantly affect
the C0 2 loading profile. Although, the sorbent having an equimolar Mg:Na ratio achieved the
lowest C0 loading of the sorbents evaluated, it exhibited peak loading at 250°C. Thus, decreasing
the sodium content of the sorbent to a Mg:Na ratio of 1:3 resulted in a significant increase in C0 2
loading at 250°C. Shifting the peak C0 2 loading from 350°C to 250°C by reducing the sodium
content of the sorbent is a significant and promising finding. This finding suggests that C0 2
interacts with the mixed salt sorbent via different mechanisms, and that the mechanism and
ultimately the window of operation can be affected by adjusting the composition.
Example 10: Effect of Precipitation Solution Concentration on Sorbent Performance
One of the preparation parameters that can affect the performance of the sorbent is the
concentration of the precipitating solution. In this study, mixed salt sorbents having the same
composition at four precipitation solution concentrations (0.05, 0.1, 0.2, and 0.3M) were prepared.
These sorbents were prepared following the same co-precipitation preparation procedure with a
Mg:Na molar ratio of 1:6. The prepared sorbents were evaluated for the removal of CO from
simulated exhaust gas in a fixed-bed reactor system at the same experimental conditions
(temperature, gas composition, and gas hourly space velocity (GHSV)) used in Example 7.
The effect of the precipitation solution concentration on the CO2 loading as a function of
temperature for the mixed salt sorbents is illustrated in Fig. 9. The concentration of the
precipitation solution has a significant effect on the performance of the sorbent. Decreasing the
concentration of the precipitating solution results in a lowering of the peak loading temperature
from 350°C to between 250 and 275°C. In addition to shifting the peak loading temperature, the
quantity of CO2 loaded increased from 12 wt% to approximately 20 wt%. The performance of the
sorbents prepared from low concentration solutions is particularly interesting, as those sorbents
achieved both greater C0 2 loading and peak loading at lower temperatures.
Example 11: Effect of Precipitating Agent on Sorbent Performance
This study evaluated the role of the precipitating agent on the performance of the mixed salt
sorbent. Two precipitating agents were evaluated: sodium carbonate (Na CO3) and ammonium
carbonate ((NH4)2CO3) . The samples were prepared using slightly different co-precipitation
techniques. The first sample was prepared by slowly adding a solution of sodium carbonate to a
solution of magnesium nitrate. The second sample was prepared by slowly adding a solution of
ammonium carbonate to a mixture of magnesium nitrate and sodium nitrate. These sorbents were
prepared with a Mg:Na molar ratio of 1:6. The prepared sorbents were evaluated for the removal of
C0 from simulated exhaust gas in a fixed-bed reactor system at the same experimental conditions
(temperature, gas composition, and gas hourly space velocity (GHSV)) used in Example 7.
The effect of the precipitating agent on the C0 2 loading as a function of temperature for the
Mg-Alkali mixed salt sorbents is provided in Fig. 10. The performance of the prepared materials is
significantly different. The sodium carbonate prepared material exhibits a broad absorption curve,
whereas the ammonium carbonate prepared sorbent has a narrower temperature range over which
absorption of C0 2 was observed with a sharp spike appearing at about 300°C. These results suggest
that the precipitating agent has an effect on the performance of the mixed salt sorbent, which may
be useful to exploit for specific applications requiring a narrow absorption temperature range.
Example 12: Compositional Analysis of Sorbents Prepared by Co-Precipitation Method
Several mixed salt sorbent compositions were prepared at different precipitation solution
concentrations according to the general process set forth in Example 2. A combination of
inductively coupled plasma (ICP) analysis and elemental analysis (CHONS) was used to
determine/estimate the molecular species composition of each prepared, co-precipitation sorbent.
The weight percentages of each component of the sorbents were estimated by combining results
from these analyses and the results are present in Table 3 below. Results indicated that reducing
the co-precipitation concentration (0.2 M 0.05 M) resulted in a higher MgO content in the
sorbent and a decrease in the NaN0 3 content. It should be noted that the sorbent materials were
prepared at a constant pH.
Table 3
Sample MgO Na2C0 3 NaN0 3
0.05 M 84.59 14.12 1.29
0.1 M 81.72 12.08 6.20
0.2 M 75.81 15.99 8.02
Example 13: Comparison of Performance of Sorbent Produced by Co-Precipitation Process and
Gelation/Colloid Process
A mixed salt sorbent of the invention was prepared having the same composition as the 0.05
M sample shown in Table 3 above using both the co-precipitation method and the gelation/physical
mixing method set forth herein, and evaluated for C0 2 capture performance at the conditions given
in Example 7. The effect of preparation method on the C0 2 capture performance of the sorbent,
having the same elemental composition, is provided in Fig. 11. It is evident that the two sorbents
have very similar C0 loading curves, with both materials achieving approximately 20 wt% C0
loading at 275°C to 300°C, with a very rapid decrease in loading with increasing absorption
temperature. The similarity in the C0 loading curves indicates that the sorbents prepared by
different preparation techniques have very similar C0 capture properties and that the desired
characteristics of the best-performing sorbent prepared by co-precipitation can be retained when
prepared by the physical mixing/gelation method.
Example 14: Effect of Magnesium Source on Sorbent Performance (Gelation Method)
As noted in Example 1, basic magnesium carbonate (magnesium carbonate hydroxide) was
the MgO source in the gelation/physical mixture sorbent. One issue with basic magnesium
carbonate is that it has very low bulk density and is more expensive than commercially-available
magnesium oxide. Two physical mixture samples were prepared using powdered MgO and nano-
MgO as the MgO source having the same composition as the sorbent prepared using basic
magnesium carbonate in Example 1 to determine if less expensive, commercially-available
magnesium oxides can be used. These sorbents were evaluated for C0 2 capture performance at the
conditions given in Example 7.
The magnesium source was found to have a significant effect on C0 loading capacity of
the mixed salt sorbent, as seen by results presented in Fig. 12. The sorbents prepared with
magnesium oxide were unable to achieve a C0 loading in excess of 5 wt C0 and showed a
decreasing C0 2 loading capacity with increasing absorption temperature. The MgO-based materials
did not show the characteristic "volcano" shape that was observed for the mixed salt sorbent. From
these results, it appears that basic magnesium carbonate should be used as the magnesium source in
the gelation process.
Example 15: Effect of Drying Method on Sorbent Performance
Two drying methods were evaluated for drying the wet sorbent material formed in the
production method to determine if the drying method affects the performance of the sorbent. The
two tested methods were: 1) oven drying of a wet filter cake; and 2) direct spray drying of the
sorbent material. Two sorbent batches having the same composition were prepared via the spray
dryer and filtering/oven drying methods. The C0 capture performance of each sorbent was
evaluated at the experimental conditions given in Example 7.
The CO2 capture performance of the sorbent prepared via the spray drying and
filtering/oven drying methods are presented in Fig. 13. These results clearly indicate that the spray
drying method yields a superior C0 2 capture sorbent. The spray-dried material exhibited ~5 wt%
higher C0 2 loading at all capture temperatures <300°C, compared to the filter/oven dried material.
However, the filtered/oven dried sorbent exhibited very good C0 2 performance as well, achieving
~ 18 wt% C0 2 loading at 300°C.
Many modifications and other embodiments of the inventions set forth herein will come to
mind to one skilled in the art to which these inventions pertain having the benefit of the teachings
presented in the foregoing descriptions and the associated drawings. Therefore, it is to be
understood that the inventions are not to be limited to the specific embodiments disclosed and that
modifications and other embodiments are intended to be included within the scope of the appended
claims. Although specific terms are employed herein, they are used in a generic and descriptive
sense only and not for purposes of limitation.
CLAIMS
1. A mixed salt composition adapted for use as a sorbent for carbon dioxide removal
from a gaseous stream, the composition being in solid form and comprising:
i) magnesium oxide;
ii) an alkali metal carbonate; and
iii) an alkali metal nitrate, wherein the composition has a molar excess of magnesium
characterized by a Mg:X atomic ratio of at least about 1.1:1, wherein X is the alkali
metal.
2. The mixed salt composition of claim 1, wherein the alkali metal comprises sodium.
3. The mixed salt composition of claim 1, wherein the Mg:X atomic ratio of at least
about 4:1.
4. The mixed salt composition of claim 1, wherein the Mg:X atomic ratio of at least
about 6:1.
5. The mixed salt composition of claim 1, wherein the alkali metal nitrate is present in
an amount of at least about 1% by weight, based on total dry weight of the mixed salt composition.
6. The mixed salt composition of claim 1, comprising the mixture
MgO:Na2C0 3:NaN0 3.
7. The mixed salt composition of claim 1, having a crush strength in pellet form, as
determined by the Shell method, of at least about 0.3 MPa.
8. A method for removing carbon dioxide from a gaseous stream, comprising
contacting a gaseous stream containing carbon dioxide with a sorbent material comprising the
mixed salt composition of any one of claims 1 to 7.
9. The method of claim 8, wherein the contacting step occurs at a temperature of about
100°C to about 450°C.
10. The method of claim 8, wherein the contacting step occurs at a temperature of about
250°C to about 375°C.
11. The method of claim 8, wherein the carbon dioxide pressure in the gaseous stream is
about 1 to about 300 psia.
12. The method of claim 8, wherein the carbon dioxide pressure in the gaseous stream
is less than about 5 psia.
13. The method of claim 8, wherein the sorbent material is contained within a fixed bed
or fluidized bed absorber.
14. The method of claim 8, further comprising the step of regenerating the sorbent
material using pressure-swing absorption, vacuum-swing absorption, temperature-swing
absorption, or a combination thereof to cause desorption of carbon dioxide.
15. The method of claim 14, wherein the regenerating step comprises at least one of
raising the temperature of the sorbent material and lowering the pressure applied to the sorbent
material, as compared to the temperature and pressure during the contacting step.
16. The method of claim 8, wherein the sorbent material exhibits a C0 2 loading of at
least about 10 weight percent C0 2 at an absorption temperature of about 250 to about 350°C.
17. A process for preparing a mixed salt composition, comprising:
i) mixing a magnesium salt with a solution comprising alkali metal ions, carbonate
ions, and optionally nitrate ions, to form a slurry or colloid comprising a solid mixed
salt comprising magnesium carbonate;
ii) separating the solid mixed salt from the slurry or colloid to form a wet cake of the
solid mixed salt;
iii) drying the wet cake to form a dry cake comprising the solid mixed salt; and
iv) calcining the dry cake to form a mixed salt composition according to any one of
claims 1 to 7.
18. The process of claim 17, wherein the mixing step comprises mixing a first solution
containing a dissolved alkali metal carbonate and optionally a dissolved alkali metal nitrate, and a
second solution containing a dissolved magnesium salt to form the solid mixed salt as a coprecipitate.
19. The process of claim 18, wherein the water solubility of the magnesium salt is at
least about 10 g per 100 ml at 25°C and one atmosphere.
20. The process of claim 18, wherein the magnesium salt is selected from magnesium
nitrate, magnesium acetate, and magnesium chloride.
2 1. The process of claim 17, wherein the mixing step comprising combining a solid
magnesium salt with a solution containing a dissolved alkali metal carbonate and a dissolved alkali
metal nitrate to form the solid mixed salt as a colloid.
22. The process of claim 2 1, wherein the solid magnesium salt is basic magnesium
carbonate.
23. The process of claim 17, wherein the carbonate ions in the solution are derived from
a precipitating salt comprising a carbonate ion added to the solution.
24. The process of claim 23, wherein the precipitating salt is an alkali metal carbonate
or ammonium carbonate.