Ĭricondenbar-the highest pressure at which a liquid and vapor can coexist.Ĭlearly, the natural gas phase envelope can be very different depending on its source.That means the mixture will be gas irrespective of pressure when the temperature is larger than cricondentherm. Ĭricondentherm-the highest temperature at which liquid and vapor can coexist.The pressure and temperature at the critical point are called critical pressure and temperature, respectively. At that given pressure and temperature, gas properties are identical to liquid properties. Ĭritical Point-the point on the phase envelope where the bubble point curve meets the dew point curve.It is the connected points of pressure and temperature at which the first liquid droplet is formed out of the gas phase. ĭew Point Curve-the curve that separates the pure gas phase from the two-phase region.This means that at a given temperature, when pressure decreases and below the bubble point curve, gas will be emitted from the liquid phase to the two-phase region. When the mixture is at the composition II, it is called an azeotrope, signifying that it boils without change of composition.Bubble Point Curve-the curve that separates the pure liquid (oil) phase from the two-phase (natural gas and oil) region. Liquid of the composition II will not be present under such conditions. If the vapour is not removed and condensed elsewhere, then the overall composition of the system does not alter, and hence the state of the system remains on the same vertical line as I, the two phases that are present obeying the lever rule. Removal of the vapour ensures that the system behaves as a one-phase (liquid phase) system. there has been a reduction in the mole fraction of A in the system by the removal of vapour that is rich in A. Note that the overall composition of the system has gone from I to II, i.e. If the vapour is removed and condensed elsewhere, then the composition of the liquid changes to one less rich in A (to take account of the excess of A in the removed vapour), for example Φ2. From the diagram, it may be seen that this vapour is richer in A than the liquid is. When so much A has been evaporated that the composition of the liquid has reached II, the vapour and liquid that are present have the same composition, and evaporation of the liquid causes no further change in composition.If a liquid of initial composition I is heated to boiling (the blue line), then vapour of composition Φ1′ is produced. Thus as evaporation of the mixture takes place, the composition of the remaining liquid increases in B as excess A is drawn off in the vapour, and the boiling point rises. If the vapour is removed and condensed elsewhere, then the composition of the liquid changes to one less rich in A (to take account of the excess of A in the removed vapour), for example Φ2.Īs this mixture boils, the vapour that is produced (of composition Φ2′, again richer in A than the liquid from which is produced) may again be removed, causing the composition of the liquid to shift to one still less rich in A. If a liquid of initial composition I is heated to boiling (the blue line), then vapour of composition Φ1′ is produced. Though many mixtures have temperature composition diagrams resembling the idealised version given above, there can be marked deviations:Ī maximum in the phase diagram may be observed if favourable interactions between the A and B molecules can stabilise the liquid phase with respect to the vapour phase: Again the relative abundance of each phase is given by the lever rule. At points between the two lines, the two phases coexist, and the composition of each phase is given by the x coordinate of the phase boundary at that temperature (just as in pressure composition diagrams). For an ideal mixture, in which A is more volatile than B, the vapour-composition diagram has the following form: The interpretation of these diagrams is in many respects entirely analogous to the interpretation of pressure composition diagrams, the only difference being that the liquid phase lies below the vapour phase, not above it: Below the lower curved line, only liquid exists, and above the upper curved line, only vapour exists. These are phase diagrams which show the composition of two phases in equilibrium at a given pressure, and how these compositions change with temperature (as opposed to the pressure composition diagrams which showed the pressure dependence of the composition at a fixed temperature).