Patent CA-1297398-C (CA)
Inventor: APFFEL FRED P
IPC: F25J3/00
Filed: 1988-04-20
Granted: 1992-03-17
Abstract: ABSTRACT      A process is disclosed for  the separation of hydrocarbon feed gas mixtures containing methane and carbon dioxide, as well as other components  such as ethane propane-and-heavier hydro-carbon, hydrogen sulfide, nitrogen, or other lighter inert gases, into  various  components.   The  steps  of  separation  include distillation  and  absorption.   These  separations  operate  at temperatures and pressures such that the freezing or formation of solid carbon dioxide will not occur.  The process utilizes four distillation units and one absorption unit.  The feed gas is fed to the first distillation system where the methane-and-lighter components   are   separated   from   the   propane-and-heavier hydrocarbons.  The top gas product from the first distillation unit is mixed with a portion of the absorbing medium methanol and refrigerated.   Part  of  the  ethane  and  carbon  dioxide  are dissolved in the methanol as a liquid.  This mixture is fed to the bottom of the absorber.  The gas containing methane, ethane and carbon dioxide flow up the absorber countercurrent to the downcoming liquid methanol.  The ethane and carbon dioxide are dissolved or absorbed by the methanol and exit the bottom of the absorber as a liquid.  The methane-and-lighter components are not dissolved or absorbed by the methanol and leave the absorber as a gas.  The bottoms from the first distillation system are fed to a second distillation column where the ethane and carbon dioxide are distilled as a top gas product.  This gas mixture and the warmed bottoms from the absorber are fed separately to a third distillation unit.  A small amount of methanol is mixed with the overhead vapor from this distillation unit.  The presence of this methanol,  a polar  substance,  causes the  relativity of carbon dioxide to ethane to both invert and the ratio to increase.  The ethane becomes the lighter component and distilled from the top of the unit as a gas.  The carbon dioxide and methanol bottoms product are fed to a fourth distillation unit where the carbon dioxide is distilled from the methanol.    Alternatively, the feed gas is fed to the first distillation system where the ethane-methane-lighter components and most of the carbon dioxide  are separated from the propane-and-heavier hydrocarbons.  The remaining carbon dioxide, propane-and-heavier hydrocarbons are fed to a second tower where the carbon dioxide is  separated  from  the  propane-and-heavier  hydrocarbons.   The carbon dioxide from this tower and the overhead gas from the first  tower  are  recombined,  compressed  and  fed  to  a  third distillation tower where 80 to 85 percent of the carbon dioxide is removed as a liquid bottoms product.  The remaining 15 to 20 percent carbon dioxide and light hydrocarbons or top gas product from this  distillation unit  is mixed with  a portion of  the absorbing medium methanol and refrigerated.  The carbon dioxide is dissolved in the methanol as a liquid.  This mixture is fed to the bottom of the absorber.  The gas containing methane, ethane and carbon dioxide flows up the absorber counter-current to the downcoming liquid methanol.  The carbon dioxide is dissolved or absorbed by the methanol and exits the bottom of the absorber as a  liquid.   The  ethane-methane-and-lighter  components  are  not dissolved or absorbed by the methanol and leave the absorber as a gas.  The carbon dioxide and methanol bottoms product from the absorber are fed to a fourth distillation unit where the carbon dioxide is distilled from the methanol.