Condensation curve

To evaluate the true temperature difference (driving force) in a mixed vapour condenser a condensation curve (temperature vs. enthalpy diagram) must be calculated showing the change in vapour temperature versus heat transferred throughout the condenser, Figure 12.48. The temperature profile will depend on the liquid-flow pattern in the condenser. There are two limiting conditions of condensate-vapour flow ... 

Differential condensation in which the liquid separates from the vapour from which it has condensed. This process is analogous to differential, or Rayleigh, distillation, and the condensation curve can be calculated using methods similar to those for determining the change in composition in differential distillation see Volume 2, Chapter 11. 

Integral condensation in which the liquid remains in equilibrium with the uncondensed vapour. The condensation curve can be determined using procedures similar to those for multicomponent flash distillation given in Chapter 11. This will be a relatively simple calculation for a binary mixture, but complex and tedious for mixtures of more than two components. 

The term dT/dHt can be evaluated from the condensation curve h c from the single component correlations and h g from correlations for forced convection. 

If this is done at several points along the condensation curve the area required can be determined by graphical or numerical integration of the expression ... 

Prepare the condensing curve, a plot of the vapor temperature Tg against the amount of heat removed Q, by a series of isothermal flashes and enthalpy balances. 

Starting at the inlet temperature Tga, specify a temperature Tg a few degrees less, and note the heat transfer AQ corresponding to this temperature difference from the condensing curve. 

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... 

Inversely, condensation begins when the temperature is decreased below the condensation curve. 

The operation of TET. First, the gas flow is cooled in heat exchanger under approximately constant pressure, then gas flow works in the turbine and is cooled further. (Note that this process is not isentropic, i.e. even in ideal case the losses are inevitable). Thus it founds itself under the condensation curve and the condensation process takes place. Further, gas again passes through heat exchanger and finally is additionally compressed by compressor. 

Hydrocarbon vapor is being condensed, using water in the tube side of a 1-2 exchanger, with no temperature cross (Fig. 2). The condensing curve is broken into three zones. In each zone, the temperature differences for the hot side and cold side are calculated, as can be seen in the table. Ibmperatures and heat duties can be read directly fi om Fig. 2. Notice, however, in the table that for the cold side, the inlet and outlet temperatures (85° and 115°F, respectively) are used for each zone. In multiple-tube-passes situations, this approximation provides a sufidciently good approach-temperature reading. 

This same calculation can be repeated for each tempoature increment along the condensation curve occurring within the heat exchanger. The results of these calculations are tabulated below ... 

Thus if D is close to the vaporization curve, the system will nearly all be in the liquid phase, while as D approaches the condensation curve more and more of the system will be vaporized. 

We shall now give a brief account of critical phenomena in the vaporization of a binary mixture. In this case it is convenient to consider the T, p diagram at constant composition (c/. fig. 16.5). For a pure substance we obtain simply the line AG which terminates at the critical point C. For a mixture of constant composition we have to consider two pressures, one corresponding to the liquid, and the other to the vapour at the same composition and temperature. These two pressures correspond to the points in fig. 13.6 where an ordinate at constant composition e.g. Xj FE) cuts the vaporization curve (G) and the condensation curve (H). (The vapour and liquid are not of course in equilibrium). As the temperature is raised the shape of the lenticular area of fig. 13.6 varies and finally decreases to zero. We thus obtain the curve FGKH of fig. 16.5 of which the branch from F to K corresponds to liquid and K to H to the vapour. The point K is the critical point at which the two phases are identical. Near to K there will also... 

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