Absorption method for recovering gas contaminants at high purity

Abstract

Solvent absorption processes for separating components of an impure feed gas are disclosed. The processes involve two stages of gas-liquid contacting, namely a first, absorption stage and a second, stripping stage. In the case of a carbon dioxide (CO2)— containing methane gas as an impure feed gas, contacting, in the stripping stage, the solvent effluent from the absorption stage with a recycled vapor fraction of the solvent effluent from the stripping stage can improve the recovery and purity of not only the methane (and / or other light hydrocarbons in the impure feed gas), but also that of the CO2 contaminant gas.

Description

FIELD OF THE INVENTION[0001]The present invention relates to gas separation methods, for example the separation of contaminant carbon dioxide (CO2) from methane using a solvent absorption process. In a two-stage, absorption / stripping operation, recycling a vapor fraction of the liquid solvent-containing stream exiting the stripper bottoms improves the purity of the recovered carbon dioxide.DESCRIPTION OF RELATED ART[0002]The removal of carbon dioxide (CO2) from predominantly light hydrocarbon- (e.g., methane-) containing, impure gas streams is desired, among other reasons, to improve the heating value of the purified gas product for pipeline transmission. Differences in a number of properties between CO2 and light hydrocarbons can serve as potential bases for gas separations. These differences include solubility, acidity in aqueous solution, and molecular size and structure. Possible separations can therefore rely on physical or chemical absorption into liquid solvents, pressure swi...

AI Technical Summary

Benefits of technology

[0008]The liquid effluent from the absorption stage nevertheless contains a significant quantity of methane, despite the relatively lower quantity of this desired, non-preferentially absorbed component. The second, stripping stage beneficially removes much of this absorbed methane into a second methane-enriched gas, which may be combined with the methane-enriched gas exiting the absorption stage. Importantly, it has been discovered that contacting, in the stripping stage, the solvent effluent from the absorption stage with a recycled vapor fraction of the solvent effluent from the stripping stage can improve the recovery and purity of not only the methane (and / or other light hydrocarbons in the impure feed gas), but also that of the CO2 contaminant gas.

[0010]Overall, therefore, aspects of the invention are associated with the significant benefits derived from “backstripping” of the first, absorption stage solvent effluent with a recycled vapor fraction (e.g., from flash separation) of the liquid effluent from the stripping stage. The benefits of conducting the second stripping stage in this manner include not only an enhanced recovery of methane, initially absorbed in the solvent after the first absorption stage, but also effective purification of the CO2 that remains absorbed in the solvent effluent after the second stage. This absorbed CO2 is subsequently recovered as CO2 product at high purity, upon regenerating the solvent at elevated temperature. In a representative embodiment, the CO2 product gas has a purity such that it is directly applicable in a number of industrial applications, including enhanced oil recovery.

Problems solved by technology

Due to the substantial heat input required to break the bonds of the heat-stable salts formed as chemical reaction products, chemical solvents are not economically regenerated.

However, the removal of a given contaminant gas using a physical solvent is limited by its partial pressure driving force.

Also, while the contaminant may be effectively removed from a desired gas such as methane, the recovery of the contaminant itself in a purified form remains problematic.

However, the CO2 contaminant is not easily recovered at high purity using conventional absorption systems, without significant additional costs.

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