Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Carbon dioxide or hydrogen sulfide component
Utility Patent
1993-12-06
2001-01-02
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Carbon dioxide or hydrogen sulfide component
C095S139000, C502S411000, C585S824000, C585S854000
Utility Patent
active
06168769
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for substantially removing carbon dioxide from a fluid containing at least one olefin where the fluid is contaminated with carbon dioxide.
BACKGROUND OF THE INVENTION
Olefin is a class of industrial chemicals useful as, for example, monomers or comonomers for the synthesis of polyolefins. However, the olefin must be substantially pure when it is used for synthesizing polyolefin, especially when the polyolefin is prepared in the presence of a high activity catalyst because any appreciable concentration of impurities can be detrimental to the catalyst. One of the impurities is carbon dioxide, a well-known catalyst inhibitor.
Alumina is a well known adsorbent in many chemical processes such as the polymerization of olefins, e.g. ethylene, for the removal of water and small concentrations of methanol, carbonyl-containing compounds, and peroxides. However, the use of alumina has disadvantages that impair its effectiveness as an adsorbent. For example, alumina has a low capacity when used as an adsorbent for the removal of CO
2
from an olefin fluid stream which contains CO
2
at low level concentrations and alumina must be regenerated when it becomes saturated with CO
2
. Incurred regeneration costs over time dramatically impact the economics of its use.
Molecular sieves are frequently used as adsorbents for CO
2
, but are inefficient when used for the removal of CO
2
from a fluid stream containing low molecular weight olefins such as ethylene.
Caustic scrubbers or bulk caustic scrubbers can also function as absorbents for CO
2
from a gaseous stream but have the disadvantages of being hazardous, and subject to water attack with subsequent caking thus severely limiting their capacity. It is therefore an increasing need to develop a more efficient process which is capable of reducing the carbon dioxide concentration in a fluid which contains an olefin to such levels that the CO
2
is not detrimental to catalyst activity in polymerization processes using the olefin.
SUMMARY OF THE INVENTION
An object of the present invention is to develop a composition for use in adsorbing carbon dioxide. Another object of the invention is to provide a process for removing carbon dioxide from a fluid stream employing the composition. An advantage of the present invention is that the composition provides a high loading capacity for adsorbing carbon dioxide. Other objects, advantages, and features will become more apparent as the invention is more fully disclosed hereinbelow.
According to the present invention, a process for removing carbon dioxide is provided which comprises contacting a fluid with a composition which comprises an oxygen-containing metal compound and an inorganic oxide compound wherein the fluid can be a gas stream, or a liquid stream, or both, and contains at least one C
2
-C
6
olefin and carbon dioxide. The process is generally carried out under conditions sufficient to remove the carbon dioxide from the fluid and the composition is present in an effective amount to substantially remove the carbon dioxide from the fluid.
DETAILED DESCRIPTION OF THE INVENTION
According to the present invention, a fluid stream containing at least one C
2
-C
5
olefin and a contaminated amount of carbon dioxide is contacted with a composition containing at least one oxygen-containing metal compound and an inorganic oxide. The olefin can be a normal or a branched olefin. The double bond can be at any position in the molecule. Examples of suitable olefins include ethylene, propylene, 1-butene, 2-butene, 2-methyl-1-butene, 1-pentene, 2-pentene, 3-methyl-1-butene, 4-methyl-1-pentene, and combinations of two ore more thereof. The presently preferred olefin is ethylene, or propylene, or both.
The oxygen-containing metal compound is generally an alkali metal compound or an alkaline earth metal compound including an alkali metal oxide, an alkali metal hydroxide, an alkaline earth metal oxide, an alkaline earth metal hydroxide, and combinations of two or more thereof. Examples of suitable oxygen-containing metal compounds include, but are not limited to, sodium oxide, sodium hydroxide, potassium oxide, potassium hydroxide, calcium oxide, calcium hydroxide, magnesium oxide, magnesium hydroxide, and combinations of two or more thereof. The presently preferred oxygen-containing metal compound is sodium hydroxide because of its ready availability and low cost.
According to the present invention, the inorganic oxide is generally the oxide used for catalyst support and is selected from the group consisting of silica, alumina, and combinations thereof. The silica employed in the invention can be any silica known in the art. The alumina employed in the invention can also be any alumina known in the art such as &agr;, &bgr;, or &dgr;-alumina.
The inorganic oxide employed generally can contain certain amount of impurities such as sodium oxide, ferric oxide, titanium oxide, and a metal sulfate.
The weight ratio of the oxygen-containing metal compound to the inorganic oxide can vary widely from as low as 0.01:1 to 10:1. Presently it is preferred that the weight ratio be in the range of from about 0.06:1 to about 9:1. The ratio is more preferably in the range of about 0.1:1 to about 5:1, and most preferably in the range of 0.15:1 to 4:1.
Generally the amount of the carbon dioxide sorbent composition used can vary widely depending on the concentration of carbon dioxide in the fluid stream to be removed as well as on the total quantity of the fluid to be treated and is an effective amount to substantially remove the carbon dioxide from the fluid. According to the present invention, the weight ratio of the carbon dioxide to the olefin is generally in the range of from about 0.000001:1 to about 0.1:1, some times 0.00001:1 to 0.01:1. Based on the contamination level of carbon dioxide in the fluid, as just described, the amount of the adsorbent required is dependent on the concentration of contaminating CO
2
in the fluid and is an effective amount for substantially removing the CO
2
. It generally can be in the range of from about 1 to about 20 g, preferably about 2 to about 10 g, and most preferably 2 to 5 g of the oxygen-containing metal compound per g of the CO
2
in the fluid to be treated, to 5% breakthrough, for carbon dioxide removal. A 5% breakthrough refers to the point in the invention process where the CO
2
concentration in the effluent fluid stream has reached 5% of the CO
2
concentration in the inlet fluid stream.
According to the present invention, the composition can be prepared by simply contacting the oxygen-containing metal compound with the inorganic oxide compound. The contacting can be carried out by any methods known in the art such as, for example, mixing or blending, as long as the oxygen-containing metal compound is well-distributed in the inorganic oxide. The composition can be used in the invention process as it is prepared or can be heated to remove moisture, if present in appreciable concentration.
The rate the fluid is contacted with the composition is not critical but can vary with reaction vessel size. Broadly, the rate of fluid introduction can be about 1 to about 30, preferably about 3 to about 20, and most preferably 5 to 10 WHSV (weight hourly space velocity; g of the fluid per g of the oxygen-containing metal compound per hour). The rate of fluid introduction can also be expressed in total volume of fluid per minute per g of the oxygen-containing metal compound and it is generally in the range from about 13 to about 400 cm
3
/min-g. The rate often varies as a function of the pressure employed in introducing to fluid which is generally in the range of from about 0 to about 2500 psig, preferably about 0 to about 2000 psig, and most preferably 0 to 1700 psig. In any event, it should be at a rate sufficient to effect efficient contact between the fluid and the composition.
According to the invention, the process can be carried out at a temperature sufficient to effect a substantial removal of the carbon dioxide from the fluid.
Evans Kerry L.
Zisman Stan A.
Griffin Steven P.
Phillips Petroleum Company
Richmond, Hitchcock, Fish & Dollar
Vanoy Timothy C
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