1. Field of the Invention
This invention relates to a novel acid gas absorbent composition. More particularly, it relates to an excellent acid gas absorbent composition for separating by absorption such acid gases as carbon dioxide and hydrogen sulfide from a mixed gas. The acid gas absorbent composition of this invention is particularly useful for the separation of acid gases which are contained in natural gas, synthetic gas, and coke oven gas.
2. Description of the Prior Art:
The practice of using organic solvents or aqueous solutions of such organic solvents as acid gas absorbents for the removal of such acid gases as carbon dioxide and hydrogen sulfide from natural gas and mixed gas has been widely known in the art. Various acid gas absorbents are mentioned in the third edition (1979) of "Gas Purification" written by A. L. Kohl and F. C. Riesenfeld, for example.
Generally, the gas absorbents are divided by the mechanism of absorption into chemical absorbents (causing absorption by chemical reaction) and physical absorbents (causing absorption by physical union). As chemical absorbents, aqueous solutions of such alkanolamines as ethanolamine and hot potassium carbonate solution are chiefly used.
In all the physical absorbents available for the removal of such acid gases as carbon dioxide and hydrogen sulfide from such mixed gases as natural gas and synthetic gas, ethers of polyethylene glycol oligomers are used most widely.
In Japanese Patent Publication SHO No. 48(1973)-23,782, U.S. Pat. No. 3,737,392, and U.S. Pat. No. 4,581,154, for example, the use of dimethyl ether of ethylene glycol oligomer (dimer to octamer) as an absorbent for the removal of acid gases from mixed gases is disclosed.
Japanese Patent Publication SHO No. 59(1984)-45,034 discloses the use of methyl isopropyl ether of polyethylene glycol of the general formula: ##STR1## wherein n is an integer in the range of 2 to 8, as an absorbent for the removal of acid gases such as carbon dioxide and/or hydrogen sulfide from a mixed gas.
Japanese Patent Laid-Open SHO 49(1974)-98,383 discloses the use of an alkylpolyethylene glycol tertiary butyl ether of the general formula: ##STR2## wherein R is a linear or branched alkyl group of one to four carbon atoms and n is an integer in the range of 2 to 10, preferably 2 to 5, as an absorbent for the removal of acid gases such as carbon dioxide and/or hydrogen sulfide from natural gas.
U.S. Pat. No. 2,139,375 discloses the use of an ether or ester or an ether-ester mixture of a polyhydric alcohol or polyhydric alcohol oligomer as an absorbent for the removal of sulfur-containing acid gases from a mixed gas. In this U.S. Patent, as examples of the absorbent applicable thereto, dipropyl ethers of diethylene glycol and dipropylene glycol are mentioned among various other substances. In the specification of this U.S. Patent, there is a mention to the effect that the ability of the absorbent to absorb the acid gases increases with the decreasing size of the alkyl group linked to the oxygen atom and methyl ether exhibits the highest ability to absorb the acid gases. It has no mention whatever about superiority of dipropyl ether.
U.S. Pat. No. 3,877,893 discloses the use of a polyethylene glycol dialkyl ether possessing one to eight carbon atoms and three to eight ethylene units as an absorbent for the removal of impurities containing carbon dioxide from a mixed gas. In the specification, polyethylene glycol dimethyl ether is designated as the best of all the specific absorbents disclosed. This U.S. Patent has absolutely no mention about speriority of diisopropyl ether.
U.S. Pat. No. 4,044,100 discloses the use of a mixture of diisopropanol amine and a polyethylene glycol dialkyl ether. Not exclusively in the specification of this U.S. Patent, there is a mention to the effect that dimethyl ether excels other ethers.
U.S. Pat. No. 4,741,745 also discloses the use of polyethylene glycol diisopropyl ether as absorbent for the removal of the acid gases.
U.S. Pat. No. 2,926,751 discloses the use of propylene carbonate as an acid gas absorbent for the removal of such acid gases as carbon dioxide from a mixed gas.
The chemical absorbents using aqueous solutions of alkanol amines have limits to the capacity for absorbing acid gases such as carbon dioxide and/or hydrogen sulfide and, therefore, prove to be disadvantageous for the treatment of a mixed gas having a high partial pressure of carbon dioxide. The aqueous solution of alkanol amine which has absorbed carbon dioxide gas is caused in a regeneration column to release the carbon dioxide gas and then returned for reuse to the carbon dioxide gas absorption column. For this regeneration, the aqueous solution must be heated. When the amount of the carbon dioxide gas absorbed in the aqueous solution is large, therefore, the heat required for the regeneration is fairly large. The susceptibility of the apparatus to corrosion by the absorbent also poses a problem.
The physical absorbent is such that the capacity thereof for absorption of carbon dioxide gas is proportional to the partial pressure of carbon dioxde gas in the mixed gas under the treatment. In the separation by absorption of carbon dioxide gas from a mixed gas having a high partial pressure of carbon dioxde gas, the ability of the physical absorbent to absorb the carbon dioxde gas is large as compared with that of the chemical absorbent. Further in the step of regeneration, the physical absorbent is easily enabled to release the absorbed carbon dioxide gas by relief of pressure of dispersion with air or other inert gas. Thus, the physical absorbent is also advantageous in terms of energy consumption over the chemical absorbent.
Such physical absorbents as dimethyl ether of polyethylene glycol and propylene carbonate heretofore disclosed to the art, however, may not well be regarded as possessing any fully satisfactory capacity for the absorption of carbon dioxide gas. When a physical absorbent possessing a still larger capacity for the absorption of carbon dioxide gas is developed, it will make a conspicuous contribution to economy as be permitting compaction of apparatus and reduction of energy consumption.
As examples of the known acid gas absorbent for the removal of acid gases such as carbon dioxide and hydrogen sulfide form mixed gases, dimethyl ether of polyethylene glycol, methyl isopropyl ether of polyethylene glycol, and alkyl polyethylene glycol-tertiary butyl ethers may be mentioned. These known acid gas absorbents, however, have no fully sufficient capacity for the absorption of acid gases.
In all the polyethylene glycol diisopropyl ethers contemplated by the invention of the our earlier development, diethylene glycol diisopropyl ether excels in ability to absorb acid gases. Since this particular compound possesses relatively high vapor pressure, the operation of absorption by the use of this compound must be carried out at a low temperature enough to avoid loss of the absorbent during the course of the operation.
An object of this invention, therefore, is to provide a novel acid gas absorbent composition.
Another object of this invention is to provide a novel physical absorbent which possesses a large capacity for the absorption of acid gases as compared with the conventional physical absorbents and possesses a sufficiently low vapor pressure, and serves advantageously for the absorption of acid gases.