An ethylene recycle method may include: obtaining an overheads stream comprising ethylene in a first gaseous state from an ethylene purification column; heating the overheads stream in a first heat exchanger to produce a heated overheads stream comprising the ethylene in a second gaseous state; compressing the heated overheads stream to yield a compressed ethylene stream comprising the ethylene in a first supercritical state; cooling the compressed ethylene stream to produce a cooled, compressed ethylene stream comprising the ethylene in a first liquid state, wherein the cooling comprises passing the compressed ethylene through the first heat exchanger; reducing the pressure of the cooled, compressed ethylene stream to produce a first recycle stream comprising the ethylene in a second liquid state and optionally a third gaseous state; and introducing the first recycle stream into the ethylene purification column.

A method for starting up vapor-recompression units or ramping up from low mass flow rates comprises: providing a vapor recompression sub-system, wherein the vapor recompression sub-system has a vapor inlet and a compressed-vapor outlet; providing a means of reducing vapor density through pressure reduction; reducing pressure within the vapor recompression sub-system to reach a sub-system pressure selected from 0.1 kPa to 1000 kPa; introducing a vapor mass flow to the vapor recompression sub-system at a restricted vapor mass flow rate; starting up the vapor recompression sub-system or ramping up from low mass flow rates while maintaining the restricted vapor mass flow rate for a time period; and then, following the time period, introducing additional vapor mass flow to the vapor recompression sub-system to reach a normal vapor mass flow rate. The restricted vapor mass flow rate is from 0.1% to 90% of the normal vapor mass flow rate.