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Currently, it is possible to transport pure ethylene in the liquid form below -103C, which is safe as long as the transport ship‘s refrigeration system keeps the ethylene well below its boiling point. If the refrigeration system stops, the ethylene will begin to boil. Once the gas escapes confinement and can mix with air, there is a chance for conflagration or explosion.
The Safety of Ethylene vs. Methane
The lower explosion limit for methane is 5% while for ethylene it is 2.75%, or approximately half that of methane. The blast radius and peak pressure of a gas cloud explosion increases with the combustible energy of the gas involved, which is related to the lower heating value (LHV). The LHV is 9530 Kcal/nm3 for methane and 15,100 for ethylene The auto-ignition temperature of methane is 580 C and for ethylene is 490 C, indicating ethylene is easier to ignite. The adiabatic flame temperature in air for methane is 3542 F and for ethylene is 3790.
Therefore, compared to methane, ethylene is easier to ignite, burns hotter, needs less oxygen to burn and produces a stronger and larger blast upon ignition of its gas cloud. Perhaps the greatest safety hazard of ethylene gas is that its density when a gas, is very close to that of air. As it boils and warms to ambient temperature, it will tend to stay at the same elevation of the release. It has a very small tendency to rise up through the air. On the other hand, methane’s density is 55% that of air, and has a strong tendency to rise in the open air away from a release. This lower density alone, greatly reduces the hazard to people and equipment of a release of methane compared to a release of ethylene.
Safe Transport of Liquid Ethylene
Agility Gas Technologies technology employs the lower hazard conditions of LNG to make ethylene transport safer. Here‘s how:
- By blending ethylene with LNG and shipping the blend in standard LNG carriers, the boiling temperature of even a 75 wt% blend is -151C. If boiling of the mixture does occur, the gas will contain just 2% ethylene by weight, not 75%. If released by accident or due to a boil off, the gas mixture would still have nearly the same inherently safer properties as the pure methane.
- If the ethylene is stored in separate holds within the tanker, the LNG, which is stored at -161C, can be used to cool the ethylene and maintain it as a liquid between -103 C and -161 C. Since ethylene does not freeze until -169C, the LNG cannot freeze pure liquid ethylene. Heat transfer systems are designed so that they can be powered or passive and inherently safe.
During transport of pure ethylene some boil off is expected. Typical values range from 2 to 10% of the cargo for today‘s ethylene transports. This loss reduces the amount of product that can be delivered. Large LNG ships have the advantage that they are bigger and the surface area relative to the volume they contain is greater. Boil off can be held within 1 to 5% or less. In general, the bigger the transport the lower the boil off losses. For a ship carrying an ethylene/LNG blend (75/25), the ethylene loss would be 2% of the 5% lost, or a total of 0.1%. For a ship carrying separately contained ethylene cooled by LNG, there would be no ethylene loss. The LNG boil off would be approximately the same as a typical LNG carrier because the boil off is directly related to the submerged area of the ship. The cost of moving liquid ethylene is currently about $250/tonne in a liquid ethylene transport. By moving ethylene in an LNG carrier, shipping charges should be in the range of $30 to $50/tonne which is typical for LNG transports.