Seaborne natural gas expansion heavy hydrocarbon recovery system and process

The invention relates to a seaborne natural gas expansion heavy hydrocarbon recovery system and a natural gas expansion heavy hydrocarbon recovery process. The system is characterized by comprising a cold box, a pre-cooling system and a heavy hydrocarbon separation system, wherein the pre-cooling system adopts three-stage refrigeration for providing cold quantity for natural gas and LPG (liquefied petroleum gas) processing in a heavy hydrocarbon separation system. When the process is adopted, the cold quantity of a pre-cooling unit can be sufficiently utilized, in addition, the self pressure energy of the high-pressure natural gas is utilized, heavy hydrocarbon in the natural gas is sufficiently recovered, the propane recovery rate is improved, and better economic performance can be reached. The internal equipment of the pre-cooling system and the heavy hydrocarbon separation system is compact and simple, flow process valve elements are fewer, and the installation is convenient, so the system and the process can be easily applied to seaborne operation. Therefore the system and the process provided by the invention can be widely applied to the heavy hydrocarbon recovery process of high-pressure natural gas of the ocean production environment.