Design of a Self-Supporting Natural Gas Liquefaction Process for Accessing Stranded off-Shore Resources

Thursday, October 20, 2011: 4:55 PM
101 D (Minneapolis Convention Center)
Achim Wechsung1, Audun Aspelund2, Truls Gundersen2 and Paul I. Barton3, (1)Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA, (2)Energy and Process Engineering, Norwegian University of Science and Technology, Trondheim, Norway, (3)Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA

Natural gas accounts for 24% of world energy consumption today, consumption is projected to grow by 44% until 2035 and natural gas is expected to become increasingly important since it combusts more cleanly and produces power more efficiently than other fossil fuels. On the one hand, close to 60% of the world's natural gas reserves are found in three countries: Russia, Iran, and Qatar. On the other hand, most of the future production growth is predicted to take place in non-OECD countries which may lead to increasingly strained and unreliable supply chains to consumers in developed nations. Stranded reserves account for the majority of the resource base and LNG will play a key role in exploiting these. Lastly, it is anticipated that public policy requiring capture and sequestration of carbon dioxide will become more widespread.

Stranded off-shore gas yields will require new technologies to make production economically viable. Traditional natural gas infrastructure for offshore production necessitates large capital expenditures to connect yields with onshore processing facilities. Ifnatural gas could be liquefied onboard dedicated platforms and then transported with LNG tankers to customers, most of the inflexible infrastructure becomes superfluous.  Additionally, since LNG can be traded much more easily, long-term contracts couldbe replaced by marketing the gas on a global gas market.

Currently, liquefying natural gas uses large amounts of energy to provide refrigeration. At the receiving terminal, LNG is re-gasified using seawater or air as a heat source. However, the "cold" of the LNG could be recycled thus improving exergy efficiency of the supply chain. For example, nitrogen and captured carbon dioxide can be liquefied providing the required heat to gasify LNG. The liquefied N2 and CO2 can be loaded aboard the empty LNG tanker which returns to the off-shore site. There, the liquefied gases will provide the required cooling to liquefy natural gas and are vented to the atmosphere (N2) or sequestered in geological formation such as depleted gas and oil fields (CO2).

While N2 and CO2 vaporize at constant temperature, natural gas condenses over a temperature interval. It is possible to modify the composite curves by adding compressors and expanders to track the condensation curve of natural gas effectively. The design goal is to find a flowsheet that is self-contained in the sense that no additional power or cooling is necessary for its operation and uses the least nitrogen.

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