Curve dewpoint

The total pressure thus has no importance. If this result is. sought from a Mollier diagram by finding the intersection of the humidity line (jr = humidity of air = constant) and the saturation curve, which gives the dewpoint temperature, a diagram constructed for a pressure of 9.S0 mbar should be used. A decent ap-proximation can be found from a diagram constructed for pressure p 1 bar. 

As previously mentioned, most onshore gas pipeline systems in the USA and Europe operate at pressures in the 500 to 100C psia range. Ground temperatures at pipeline depth approach J32°F in winter In many areas, so it is possible for the gas temperature to approach this level. Both of the gas streams whose dewpoint curves are shown on Figure 1 would oondense liquids at such conditions. In the North Sea, however, the environmental conditions will be different, and we also expect that gas pipeline systems will be designed to operate at higher pressure levels. 

While these curves apply only to the specific conditions listed, some generally applicable obaerva-atlons should be made. With an inlet pressure of 2425 psla a distance of at least 200 miles would be required for the gas to reach dewpoint conditions for the higher dewpoint gas shown (49°F at 1000 psla). 

The culprit is the phenomenon of retrograde condensation, which wa6 previously discussed In connection with hydrocarbon dewpoints. This can best be understood by looking at a graph of equilibrium ratios, commonly called K-values, as shown in Figure 2. We have cross-plotted a limited number of curves, to avoid confusion while Illustrating our point. 

A mixture with initial dewpoint 139.9°C and final bubblepoint 48.4°C is to be condensed with coolant at a constant temperature of 27°C. The gas film heat transfer coefficient is 40 W/m2 K and the overall coefficient is 450. Results of the calculation of the condensing curve are... 

Figure 10 shows the relationship between yx and xx for different values of an calculated from Eq. (8). When two components have close boiling points, by implication they have similar vapor pressures, so that an is close to unity. Separation of mixtures by distillation becomes more difficult as an approaches unity. Figure 11 indicates some of the x, y diagrams that can be obtained for distillation systems. Also shown are corresponding temperature-composition diagrams. The saturated vapor or dewpoint curve is determined by finding the temperature at which liquid starts to condense from a vapor mixture. Similarly, the saturated liquid or bubble-point curve corresponds to the temperature at which a liquid mixture starts to boil. For ideal mixtures, the dewpoint and bubble-point curves can be calculated as follows. From Eq. (3), at the dew point, since... 

This behavior requires positive deviations from Raoult s law over part of the composition range and negative deviations over the remainder. Thus a plot of GE vs. x starts and ends with GE = 0 at A i = 0 and X] = 1 and shows positive values over part of the composition range and negative values over the remainder, with an intermediate crossing of the Xi axis. Because these deviations are usually quite small, the vapor pressures P 1 and P2sat must not be too different, otherwise the dewpoint and bubblepoint curves cannot exhibit extrema. 

Curve ABC in each figure represents the states of saturated-liquid mixtures it is called the bubble-point curve because it is the locus of bubble points in the temperature-composition diagram. Curve ADC represents the states of saturated vapor it is called the dewpoint curve because it is the locus of the dew points. The bubble- and dew-point curves converge at the two ends, which represent the saturation points of the two pure components. Thus in Fig. 3.6, point A corresponds to the boiling point of toluene at 133.3 kPa, and point C corresponds to the boiling point of benzene. Similarly, in Fig. 3.7, point A corresponds to the vapor pressure of toluene at 100°C, and point C corresponds to the vapor pressure of benzene. 

For the mixture isopropyl ether/isopropyl alcohol, positive deviations from Raoult s law occur. They are so strong that the total pressure reaches a maximum for a certain concentration. The vapor pressure maximum corresponds to a boiling temperature minimum as it is depicted in the middle row. Boiling curve and dewpoint curve contact each other in the so-called azeotropic points. Here the mole fractions of vapor y and liquid have the same value which also follows from the equilibrium diagram below. In the azeotropic point A the equilibrium curve intersects the diagonal Pa = To the left of point H the mole fraction of compo-... 

Consider the P-T diagram of a hydrocarbon mixture when the interface between the phases is flat (see the sketch below). Now suppose the interface between the phases is curved with the gas on the convex side. The dewpoint curve is sketched below for the curved interface system. Sketch the bubblepoint curve of the same system with a curved interface. 

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