NG　(Natural　gas)　distribution　pressure　varies　in　a　wide　range　in　different　applications　and　has　significant　effect　on　system　exergy　efficiency.　RC　(Rankine　cycle)　and　DEC　(direct　expansion　cycle)　can　utilize　the　cold　and　pressure　exergies　of　LNG　(liquefied　natural　gas),　respectively.　In　this　paper,　SRC　(single-stage　RC),　PTRC　(parallel　two-stage　RC)　and　CTRC　(cascade　two-stage　RC)　with　and　without　DEC　are　optimized　by　Particle　Swarm　Optimization　and　compared　to　obtain　the　optimal　configurations　for　different　NG　distribution　pressures.　Propane,　propylene,　ethane　and　ethylene　are　adopted　as　the　working　fluids.　The　results　show　that　the　addition　of　DEC　could　increase　the　exergy　efficiency　by　lifting　the　evaporation　temperature　of　LNG.　CTRC　+　DEC　with　propane/ethylene　and　CfRC　+　DEC　with　propane/ethane　as　the　working　fluid　achieve　the　largest　exergy　efficiencies　of　23.89%　and　18.25%　at　NG　distribution　pressures　of　1　MPa　and　2　MPa,　respectively.　PTRC　+　DEC　with　propane/ethane　is　the　optimal　system　with　maximum　exergy　efficiencies　of　15.18%,　13.24%,　11.7%　and　10.4%　at　NG　distribution　pressures　of　3,　4,　5　and　6　MPa,　respectively.　PTRC　+　DEC　with　ethane　as　the　working　fluid　in　both　cycles　is　a　generally　suitable　system　for　all　NG　distribution　pressures　with　high　efficiency　and　only　one　type　of　working　fluid.　(C)　2017　Elsevier　Ltd.　All　rights　reserved.