Processes for viscosity breaking of plastics

A system for converting solid plastic waste comprises:

• a melting vessel;
• a visbreaker unit in fluid communication with the melting vessel and downstream of the melting vessel, wherein the visbreaker unit comprises a visbreaker furnace and a soaker vessel downstream of the visbreaker furnace;
• a visbreaker effluent separation unit in fluid communication with the visbreaker unit and downstream of the visbreaker unit;
• and
• a supercritical water unit in fluid communication with the visbreaker effluent separation unit and downstream of the visbreaker effluent separation unit, wherein the supercritical water unit comprises:
• a mixing unit;
• a pressurized water preheater upstream of and in fluid communication with a tubular reactor,
• the tubular reactor being downstream of the mixing unit;
• and
• a furnace, wherein:
• the furnace is the visbreaker furnace;
• the at least a portion of the tubular reactor passes through a convection section of the visbreaker furnace;
• and
• at least a portion of the pressurized water preheater passes through a radiation section of the visbreaker furnace
• wherein:
• the visbreaker furnace is in fluid communication with the soaker vessel via a heated liquid plastic stream,
• the system further comprises a stripping gas stream in fluid communication with the heated liquid plastic stream at a point between the visbreaker furnace and a stripper vessel;
• and
• the stripping gas stream passes through the convection section, upstream of the point of fluid communication between the stripping gas stream and the heated liquid plastic stream.