Carbon dioxide adsorbents containing magnesium oxide...

Details Carbon dioxide adsorbents containing magnesium oxide... Carbon dioxide adsorbents containing magnesium oxide...

1999-08-06

2001-08-28

Smith, Duane (Department: 1724)

Gas separation: processes

Solid sorption

Including reduction of pressure

C095S097000, C095S104000, C095S121000, C095S122000, C095S139000, C095S148000

Reexamination Certificate

active

06280503

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
Not applicable.
FIELD OF THE INVENTION
This invention relates to a process for adsorbing carbon dioxide from a gas stream at temperatures ranging form 300 to 500° C. which utilizes adsorbents containing magnesium oxide. In another aspect it relates to a promoted magnesium oxide adsorbent having a unique composition.
BACKGROUND OF THE INVENTION
For most of the twentieth century the removal of carbon dioxide from other gases has been the subject of considerable industrial research. Many processes have been developed for this purpose. The most common methods for separating out carbon dioxide involve selective adsorption, often with reversible chemical reactions of the carbon dioxide with chemicals in the sorbent. Well known methods involve bubbling a gas stream containing carbon dioxide through caustic liquids, such as alkanol amine solutions or solutions containing soda, ammonia and/or carbonates, that adsorb CO
2
through the formation of metal carbonate salts, such as Na
2
CO
3
. A process of this type is described in U.S. Pat. No. 1,831,731, J. Al, (1931) which discloses reacting CO
2
with K
2
CO
3
in the presence of water to form KHCO
3
. The alkali carbonate can be regenerated with ammonia. These liquid systems are costly, tend to entrain liquid in downstream equipment, generally require high maintenance, and are practical only at moderate temperatures, e.g. 20 to 50° C.
A number of solid state adsorbents have been developed to avoid the above-mentioned problems associated with liquid systems. For example, U.S. Pat. No. 3,141,729, Clarke et al., (1964) describes removing CO
2
from an atmosphere with a cogel of a lithium or divalent metal oxide with a trivalent metal oxide, such as MgO. Al
2
O
3
. Also, U.S. Pat. No. 5,520,894, Heesink et al., (1996) discloses removing CO
2
from a hot gas stream such as flue gas with a solid absorbent of CaO, MgO or CaCO
3
. MgO. These systems require elaborate operating procedures, such as preprocessing the gas streams to remove competitive species, or extreme conditions of regeneration.
Several patents describe the removal of CO
2
from gas streams with adsorbents supported on alumina. U.S. Pat. No. 3,511,595, Fuchs, (1970) discloses the removal of CO
2
by reaction with an alkali metal carbonate coated on or impregnated in a high surface area carrier such as alumina. U.S. Pat. No. 3,865,924, Gidaspow et al., (1975) describes removing CO
2
with an alkali metal carbonate ground together with alumina. U.S. Pat. No. 4,433,981, Slaugh et al., (1984) discloses removing CO
2
with a calcined oxide or decomposable salt of an alkali metal or alkaline earth metal impregnated on a porous alumina support. U.S. Pat. No. 4,493,715, Hogan et al., (1985) discloses removing CO
2
from an olefin stream using a calcined alkali metal compound on alumina. In each of these patents, regeneration of the adsorbent is accomplished by heating in a temperature swing operation.
Although not part of the CO
2
adsorption art, a description of complex or double salts of magnesium carbonate is given in the text,
A Comprehensive Treatise on Inorganic and Theoretical Chemistry,
J. W. Mellor, John Wiley & Sons, N.Y., Vol. 4, pp. 367-376 (1960). This reference discloses that heating crystalline double salts of alkali metal and magnesium carbonate, e.g. K
2
Mg(CO
3
)
2
.4H
2
O or KHMg(CO
3
)
2
.H
2
O, results in evolution of CO
2
leaving a mixture of MgO and K
2
CO
3
. Although these carbonate double salts containing alkali metals and magnesium in stoichiometric proportions have been known for several years, there is no known reference to non-stoichiometric double salts nor to the study or use of this family of materials as CO
2
adsorbents.
U.S. Pat. No. 5,454,968, Nalette et al. (1995) which, while not referring to their compositions as double salts, describes the formation of pastes of mixtures of metal carbonate and alkali metal carbonate that can be formed into flat sheets which are used to sorb carbon dioxide from gas streams flowing through the sheets. The pastes are prepared by forming an aqueous solution of alkali metal carbonate, e.g. K
2
CO
3
, which is blended with a powder of metal carbonate, preferably silver carbonate. Other metals mentioned are zinc and magnesium. The paste is then formed into a flat sheet, constrained between screens, and heated to drive off water and to convert the silver carbonate to silver oxide. The CO
2
and H
2
O are said to react with K
2
CO
3
during operation to form KHCO
3
which then reacts with AgO to form AgCO
3
and K
2
CO
3
plus H
2
O. Regeneration is accomplished by heating, e.g. to 160-220° C., to liberate CO
2
from the silver carbonate. The maximum operating temperature is said to be 250° C. The sorbents of Nalette et al. have been found in practice not to be totally satisfactory, particularly in high temperature operations where regeneration of the adsorbent must be accomplished by thermal swings rather than by pressure swings. Therefore, further improvements in CO
2
adsorbents is highly desirable.
BRIEF SUMMARY OF THE INVENTION
According to our invention a process is provided for removing carbon dioxide from a gas stream at temperatures in the range of 300 to 500° C. using magnesium oxide-containing adsorbents represented by the general chemical formula:
{(M
2
CO
3
)
m
(2MHCO
3
)
(1−m)
}
n
(MgCO
3
)
p
(MgO)
(1−p)
.xH
2
O
wherein M is alkali metal, 0≦m≦1, 0≦n≦1.30, 0≦p≦1 and x represents the extent of hydration of the adsorbent, with the further provision that when n equals 0 the MgO is made by the dehydration and CO
2
removal of {MgCO
3
}
y
.{Mg(OH)
2
}
(1−y)
.xH
2
O, in which 0.1≦y≦0.9, and x indicates the extent of hydration. Adsorbents represented by this formula which contain an alkali metal salt and a magnesium salt are referred to as double salts. As is apparent from this formula, the adsorbents always contain some MgO plus either additional MgO or MgCO
3
, depending upon the stage in the process of CO
2
adsorption. Further, the adsorbent may or may not contain alkali metal which has been found to promote the efficiency of the adsorbent. The atomic ratio of alkali metal to magnesium is always in the range of 0 to 2.60. Preferably the alkali metal is present in an atomic ratio of alkali metal to magnesium of at least 0.006, and more preferably of at least 0.08.
During operation alkali metal carbonate may be converted to the bicarbonate as indicated by the formula given above, but whether the alkali metal is present in the form of carbonate or bicarbonate, the atomic ratio of alkali metal to magnesium cannot exceed 2.60. The normal stoichiometric ratio of alkali metal to magnesium in such double salts is 2.0 as disclosed by Mellor, cited in the Background section. We believe these non-stoichiometric compositions are novel and, as shown by the data of the Examples, provide clear advantages when used in processes for reversibly adsorbing CO
2
from CO
2
-containing gaseous mixtures under temperatures ranging from 300°-500° C. For purposes of interpretation, the term, non-stoichiometric, means that n of the enumerated formulae does not equal zero.
Another aspect of our invention is a non-stoichiometric double salt of alkali metal and magnesium represented by the formula
{(M
2
CO
3
)
m
(2MHCO
3
)
(1−m)
}
n
(MgCO
3
)
p
(MgO)
(1−p)
.xH
2
O
wherein M is alkali metal, 0≦m≦1, 0.003≦n≦0.925, 0≦p≦1 and x represents the extent of hydration. The atomic ratio of M to Mg is in the range of 0.006 and 1.85, preferably at least 0.08.


REFERENCES:
patent: 1831731 (1931-10-01), Al
patent: 3141729 (1964-07-01), Clarke
patent: 3232028 (1966-02-01), McDonald et al.
patent: 3489693 (1970-01-01), Bovard
patent: 3511595 (1970-05-01), Fuchs
patent: 3865924 (1975-02-01), Gidaspow et al.
patent: 4433981 (1984-02-01), Slaugh et al.
patent: 4482361 (1984-11-01), Whysall
patent: 4493715 (1985-01-01), Hogan et al.
patent: 4508690 (1985-04-01), Obrist et al.
patent: 4656156 (1987-04-01)

Affiliated with

Also associated with

Rating

Carbon dioxide adsorbents containing magnesium oxide... does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Carbon dioxide adsorbents containing magnesium oxide..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Carbon dioxide adsorbents containing magnesium oxide... will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2479829