Liquefaction of Natural Gas in Offshore Installations: Effects of Irreversibilities and Composition of Natural Gas

Abstract

The use of natural gas in the energy matrix has been gaining prominence on the world scenery, due to being a fuel with available reserves and being less pollutant than other fossil fuels. Gas pipelines can be hard to build when oil and gas production takes place in deep waters and at a great distance from the coast. An alternative is to liquefy natural gas at the production site (LNG). The LNG can then be transferred to land by methane tankers. Offshore natural gas liquefaction is strongly limited by weight and available space on vessels. In this work, three natural gas liquefaction processes are evaluated (Joule-Thomson cycle, reverse Brayton cycle and Claude cycle). The main objective is to evaluate the sensitivity of each type of technology to the main irreversibilities present: isentropic efficiencies, pressure drop and minimum temperature differences in heat exchangers. Each system is modeled in the ASPEN-Hysys environment, with Peng-Robinson equations of state. After validating the modeled systems, sensitivity studies are performed on the main sources of irreversibility for each system. The effect of the composition of the natural gas to be liquefied on the performance of the systems was also analyzed. The obtained results showed that design parameters (and the irreversibilities associated with them) produce enormous effects on the performance of the liquefaction systems, indicating that compromises between weight and space available in the FPSO may imply the adoption of non-optimized systems in terms of exergy efficiency.