Lights removal from carbon dioxide
Light gases such as helium are extracted from a carbon dioxide-containing feed stream by distillation. Costly dehydration steps are avoided by pumping the liquid bottoms stream leaving the distillation column without vaporization so as to ensure that any water present in the feed remains in solution with the bulk stream leaving the process. This prevents any liquid phase water causing corrosion or solid ice or hydrates forming to plug the flow.
1. A process for recovering a light gas from a feed stream comprising carbon dioxide and at least one light gas selected from the group consisting of helium, methane, nitrogen, argon, and oxygen, said process comprising:
cooling said feed stream to form a distillation column feed stream;
separating said distillation column feed stream in a distillation column system to produce a lights-enriched vapor and a lights-depleted bottoms liquid;
pumping said lights-depleted bottoms liquid to produce a pumped lights-depleted stream;
heating said pumped lights-depleted stream without vaporizing to produce a warmed lights-depleted stream;
cooling and at least partially condensing the lights-enriched vapor stream;
separating the cooled lights-enriched stream in a condenser separator to produce a crude lights stream and a condensed stream;
feeding said condensed stream to said distillation column system;
warming the crude lights stream by indirect heat exchange with a recycle stream, producing a warmed crude lights stream and a cooled recycle stream;
and feeding said cooled recycle stream to said distillation column system.
2. The process of claim 1 wherein at least a portion of said condensed stream enters the distillation column at a higher stage than the stage at which the distillation column feed stream enters the distillation column.
3. The process of claim 1 wherein the feed stream is in the liquid phase and below the critical pressure of the feed stream.
4. The process of claim 1 wherein the feed stream is above the critical pressure of the feed stream.
5. The process of claim 1 further comprising the step of pumping said warmed lights-depleted stream to produce a lights-depleted return stream.
6. The process of claim 5 wherein the feed stream comprises a condensable compound selected from the group consisting of water, mercury, and heavy hydrocarbons.
7. The process of claim 6 wherein the lowest minimum solubility safety factor for condensable compounds among the lights-depleted bottoms liquid, the pumped lights-depleted stream, the section of the main heat exchanger in which the pumped lights-depleted stream is heated to form the warmed lights-depleted stream, the warmed lights-depleted stream, and the lights-depleted return stream is greater than the lowest minimum solubility safety factor for condensable compounds among the distillation column feed stream, the lights-enriched vapor, the cooled lights-enriched stream, and the distillation column system.
8. The process of claim 1 wherein the feed stream is expanded after cooling and prior to separation in said distillation column system.
9. An apparatus for carrying out the process of claim 1 , said apparatus comprising:
said distillation column system for separating said distillation column feed stream to produce said lights-enriched vapor and said lights-depleted bottoms liquid;
a pumping system in fluid flow communication with said distillation column system for pumping said lights-depleted bottoms liquid to produce said pumped lights-depleted stream;
a first heat exchanger system in fluid flow communication with said distillation column system and said pumping system for cooling said feed stream to produce said distillation column feed stream by indirect heat exchange against said pumped lights-depleted stream without vaporizing to produce said warmed lights-depleted stream;
a second heat exchanger system in fluid flow communication with said distillation column system for the cooling and at least partial condensation of said lights-enriched vapor;
the condenser separator in fluid flow communication with said second heat exchanger system and said distillation column system for the separation of said cooled lights-enriched stream to produce the crude lights stream and the condensed stream;
a purification system for separating said crude lights stream to produce a pure lights product and the recycle stream;
and a third heat exchanger system in fluid flow communication with said condenser separator and said purification system for the heating of said crude lights stream by indirect heat exchange against said recycle stream.