This invention is concerned with the separation of a mixture of carbon dioxide and ethane derived from a prior separation or recovery stage or process. More particularly, the invention pertains to separation of a carbon dioxide-ethane azeotrope by freezing the carbon dioxide and removing the ethane with propane or a heavier hydrocarbon.
It is sometimes desirable to separate carbon dioxide from a gaseous mixture containing ethane, for example, in the treatment of natural gas or a reservoir injection gas, or in the preparation of synthetic natural gas. In one or more stages of these processes, carbon dioxide, ethane and heavier hydrocarbons are separated or recovered as a mixture. It is then desirable to separate the carbon dioxide from the remaining hydrocarbons, sometimes called NGL (natural gas liquids). When carbon dioxide is distilled or fractionated from the hydrocarbons, an azeotrope of carbon dioxide and ethane is formed at an overhead point in the distillation column. At this point no more ethane is separated from the carbon dioxide. Generally, unless it is desirable to recover the carbon dioxide, the carbon dioxide is vented unless the concentration of ethane in the azeotrope is such that environmental regulations make it necessary to burn the ethane-carbon dioxide mixture. Flaring is frequently an expensive and difficult process, especially in areas where the amount and concentration of the gas being flared randomly varies with various operating or producing conditions. In processes where it is desirable to recover the carbon dioxide, for example, tertiary recovery processes, the unseparated ethane is lost.
The ethane in the carbon dioxide is a valuable hydrocarbon. This invention is concerned with separating and recovering ethane along with the other hydrocarbons that might be in a carbon dioxide-ethane mixture derived from a prior process.