tl;dr: No, shipping plastic waste to China for recycling does not offset the benefits, as it reduces the net energy savings by 3% or less.
Polyethylene, The most common plastic
According to Wikipedia, polyethylene is the most common type of plastic. There are three main categories of polyethylene (also from Wikipedia):
- PET (polyethylene terephthalate), used in polyester fibers, drink bottles, tote bags, furniture, etc.
- HDPE (High-density polyethylene), used in bottles, playground equipment, plastic lumber, etc.
- LDPE (Low-density polyethylene), used in plastic bags, six pack rings, tubing, etc.
Oil and gas for PE production
PE is made from ethylene, which can be produced from either natural gas or crude oil. Since ethylene production also generates other useful products, the amount of oil or gas specifically used to produce PE is a bit tricky to track down. However based on the global production of ethylene, PE production uses less than 3% of global crude oil production, and less than 4% of global natural gas production.
Energy savings from PE recycling
More significant, and easier to track, is the amount of energy used in PE production.
A 2003 life cycle assessment from the University of Naples found that recycling of PE takes from 40 to 55 MJ per kg, while production from raw materials requires 77 to 80 MJ per kg. The difference is a savings of 22 to 40 megajoules (MJ) per kg (roughly 6 to 11 kWh/kg). This study is specific to Italy, and unfortunately the details are behind a paywall.
In 2010 the American Chemistry Council (ACC), the Association of Postconsumer Plastic Recyclers (APR), the National Association for PET Container Resources (NAPCOR), and the PET Resin Association (PETRA) funded a life cycle analysis of PET and HDPE recycling. The study is available here: "Life Cycle Inventory of 100% Postconsumer HDPE and PET Recycled Resin from Postconsumer Containers and Packaging".
The report provides the following data on energy use of recycled vs "virgin" plastic production. These figures are for the U.S., and include the energy used to collect, transport, and process the plastic after its first use.
- Raw HDPE takes 35.8 MMBtu per 1,000 lbs (23 kWh/kg), while recycled HDPE requires 3.87 to 4.24 MMBtu per 1,000 lbs (2.5 to 2.7 kWh/kg), depending on whether collection is allocated by weight or volume. This is a savings of roughly 20 kWh/kg.
- Raw PET takes 31.9 MMBtu per 1,000 lbs (20.6 kWh/kg), while recycled PET requires 5.18 to 5.45 MMBtu per 1,000 lbs (3.3 to 3.5 kWh/kg). This is a savings of roughly 17 kWh/kg.
Shipping across the Pacific
Assuming an average speed of 15 knots, energy usage is between 0.004 and 0.008 kWh per nautical mile per tonne (kWh/n.m./DWT) for larger classes of ships (source).
Depending on the ports and route, shipping from the US to China involves a journey of 6,000 to 11,000 nautical miles.
Thus shipping plastic from the U.S. to China will require from 24 to 88 kWh/tonne.
Energy savings from recycling: 6,000 to 20,000 kWh/tonne
The specific energy savings from PE recycling depends on the type of PE and the allocation method used (whether by weight or volume).
Energy consumption for shipping: 24 to 88 kWh/tonne
This value depends on the vessel, speed, and route between the U.S. and China.
- In the worst-case scenario (including shipping the plastic back to the U.S.), shipping plastic for recycling reduces the energy savings by 2.9% .
- In the best case scenario, savings are reduced by 0.12%.
Recycling plastic saves so much energy (a good thing!) and shipping things across the ocean uses so little energy (also a good thing!), that shipping our dirty plastic to China for recycling isn't actually terrible.