Bubble-point type curve

A third fundamental type of laboratory distillation, which is the most tedious to perform of the three types of laboratory distillations, is equilibrium-flash distillation (EFV), for which no standard test exists. The sample is heated in such a manner that the total vapor produced remains in contact with the total remaining liquid until the desired temperature is reached at a set pressure. The volume percent vaporized at these conditions is recorded. To determine the complete flash curve, a series of runs at a fixed pressure is conducted over a range of temperature sufficient to cover the range of vaporization from 0 to 100 percent. As seen in Fig. 13-84, the component separation achieved by an EFV distillation is much less than by the ASTM or TBP distillation tests. The initial and final EFN- points are the bubble point and the dew point respectively of the sample. If desired, EFN- curves can be established at a series of pressures. 

In Figure 2.2-7a the bubble-point curve shows a horizontal point of inflection at the critical point l2=h and in Figure 2.2-7d the binodal shows a horizontal point of inflection at the critical point lj-g. At temperatures lower than TLcep and temperatures higher than Tucep the P c-sections are the same as for type I systems. 

In the systems that we have examined so far, the bubble point and the dew point of the mixture vary monotonically with the composition. This is the case for ideal systems. However, for very non-ideal systems, there may be a maximum or a minimum in the bubble and dew point curves. This is the case for azeotropic systems. An example of a system that exhibits a low-boiling azeotrope is a mixture of 77-heptane and ethanol, which is shown in Figure 3.5. For this type of system, both the bubble and dew point temperature curves have a local minimum at the same composition. At this composition, these two curves meet. This point is known as the azeotrope. At the azeotrope, the composition of the coexisting liquid and vapor phases are identical. In this case at the azeotrope, the boiling temperature... 

The discussion of the previous section was concerned with low-pressure vapor-liquid equilibria and involved the use of activity coefficient models. Here we are interested in high-pressure phase equilibrium in fluids in which both phases are describable by equations of state, that is, the cj -4> method. One example of the type of data we are interested in describing (or predicting) is shown in Fig. 10.3-1 for the ethane-propylene system. There we see the liquid (bubble point) and vapor (dew point) curves for this system at three different isotherms. At each temperature the coexisting vapor and liquid. phases have the same pressure and thus are joined by horizontal tie lines, only one of... 

By simply knowing the phase equilibrium behavior and the composition within the beaker at the start of the experiment (x ), one can easily construct a residue curve by integrating Equation 2.8. Such integration is usually performed with the use of a numerical integration method (see later, Section 2.5.3), such as Runge Kutta type methods, remembering that at each function evaluation, a bubble point calculation must be performed in order to determine y(x). 

The equilibrium curve, or IC-value, is for the most part represented by a straight line with slope K (or K,). Since this pertains to the more-permeable component, the slope is greater than unity. Furthermore, the K-value line lies above the 45° diagonal, albeit the actual determination is to a certain extent arbitrary, as per Example 3.1 of Chapter 3 that is, is it determined from a bubble-point, dew-point, or in-between type calculation on the feedstream composition ... 

Figure 5.1 la-c plot the bubble point pressure dependence on the liquid type. The black line is the prediction curve based on room temperature pore diameters. Pure liquid test results from Chapter 4 are plotted along with NBP LH2 and LN2 data from the current work. Again, to isolate the liquid dependence, only cryogenic data collected using GHe to pressurize the screen is plotted. 

The different characterizations of the heavy ends, which make up less than 1% of the entire mixture, were considered. In Case 1 normal butane was selected to represent the heavy ends, in Case 2 normal pentane, and in Case 3 normal hexane. The phase envelopes for the three cases were predicted with the BWR—11 equation of state. The results are given in Figure 14. The bubble-point curves almost coincide, and the true critical points are not very sensitive to composition. However, as shown, the dew-point is greatly affected by the characterization of the heavy ends composition, and there is about a 70 F difference in the maximum dew-points of the three mixtures. Therefore, when working with a natural gas type system, if the breakdown of the C+ fraction into additional compounds is available, it should be used, especially in flash calculations, to get an accurate representation of the phase behavior. 

This type of bubble-point calculation has no simple graphical solution on the four types of plots shown in the previous examples because the pressure is unknown. Figure 8.17 shows a T-x diagram for ethanol-water at 1 atm, similar to part d in Figures 8.7, 8.8, 8.9, and 8.12 for a pressure of 1.00 atm. In principle, we could have a separate plot of this type for each possible pressure. (If we wanted the equivalent of Figure 8.17 for some other pressure we could make it up by repeating Example 8.10 (below) at that pressure, for a variety of liquid compositions and plotting the results.) With a set of such plots we would try to find the plot on which the specified T and Xa lie exactly on the liquid composition curve. Then we would read the pressure at which that plot was made, and the at that temperature. In... 

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