Vapor-composition curve

For meaningful data to be obtained, only the center portion of the liquid-vapor composition curve should be used in this calibration. Therefore, the calibrating mixture should be chosen with the probable number of theoretical plates in mind. The values of n in the last column of Table 1-10 are those which will produce a distillate hav-... 

Minimum-boiling mixtures—azeotropes When the positive deviations from ideality are sufficiently large and the vapor pressures of the two components are not too far apart, the total-pressure curves at constant temperature may rise through a maximum at some concentration, as in Fig. 9.1a. Such a mixture is said to form an azeotrope, or constant-boiling mixture. The significance of this is more readily seen by study of the constant-pressure section (Fig. 9.1b or c). TTie liquid- and vapor-composition curves are tangent at point L, the point of... 

Hie solution CO2 loading drops rapidly in the column immediately below the feed point, then decreases more slowly until the bottom of the column is reached, and finally exits the reboiler with a CO2 loading of 0.12 mole/mole. As would be expected, the liquid and vapor composition curves have the same shape. Above the feed point the v or phase is about one-third CO2 and two-thirds water vqtor, lepresentiiig a reflux ratio of 2 1 mole H20/mole CO2. These results can be compared t Figore 2-81, which also shows vapor leaving the stripping section of an MEA CO2 unit with a ratio of about 2 1 mole H20/mole CX>2. 

The composite curves for this flowsheet are shown in Fig. 14.86. The composite curves are dominated by the reboilers and condensers of the two distillation columns and the feed vaporizer for the acetone feed. It is immediately apparent that the two distillation columns are both inappropriately placed across the pinch. Linnhoflf and Parker ... 

A composite curve of heat of infinite dilution of oleum from reported data (3,88—90) is presented in a compiled form in the Hterature (91), where heats of formation of oleums from Hquid or gaseous SO are also reported (Tables 5 and 6). Heat of vaporization data are also available (92). Oleum heat capacity data are presented in Figure 18 (76) solubiUty data for SO2 in oleum can be found in Reference 69. 

Figure 8.17 Vapor fugacity for component 2 in a liquid mixture. At temperature T, large positive deviations from Raoult s law occur. At a lower temperature, the vapor fugacity curve goes through a point of inflection (point c), which becomes a critical point known as the upper critical end point (UCEP). The temperature Tc at which this happens is known as the upper critical solution temperature (UCST). At temperatures less than Tc, the mixture separates into two phases with compositions given by points a and b. Component 1 would show similar behavior, with a point of inflection in the f against X2 curve at Tc, and a discontinuity at 7V...

FIGURE 8.37 A temperature-composition diagram for benzene and toluene. The lower, blue curve shows the boiling point of the mixture as a function of composition. The upper, orange curve shows the composition of the vapor in equilibrium with the liquid at each boiling point. Thus, point B shows the vapor composition for a mixture that boils at point A. 

For matches between process and refrigeration, A Tmin = 10°C. Draw the process grand composite curve and set the targets for the utilities. Below the pinch use of higher temperature, cold utilities should be maximized. For boiler feedwater, the specific heat capacity is 4.2 kJ kg K-1 and the latent heat of vaporization is 2238 kJ-kg1. 

To understand how the diagram works, let s imagine starting with the 50 50 benzene/toluene mixture and heating it to its boiling point (92.2°C on the diagram). The lower curve represents the liquid composition (50 50), but the upper curve represents the vapor composition (approximately 71 29 at 92.2°C). The two points are connected by a short horizontal line called a tie line to indicate that the... 

FIGURE 11.18 A phase diagram of temperature versus composition (mole fraction) for a mixture of benzene and toluene. Liquid composition is given by the lower curve, and vapor composition is given by the top curve. The thin region between curves represents an equilibrium between phases. Liquid and vapor compositions at a given temperature are connected by a horizontal tie line, as explained in the text. 

Pressure has a marked effect on the azeotropic composition and vapor-liquid equilibrium diagrams of alcohol-ketone systems (J). This is due to the fact that the slopes of the vapor pressure curves of alcohols are appreciably greater than for ketones it results in an unusually larger change in the relative boiling points of the components of an alcohol-ketone system with change in pressure. 

Another use for this set of curves is for estimating the azeotropic boiling point and composition at pressures other than atmospheric. Consider the azeotrope methanol-benzene. Since the vapor pressure curves of methanol and benzene are known, the difference in boiling point, A, can be obtained at any pressure. From this value of A and the C-A curve for methanol-hydrocarbons the azeotropic concentration C at that pressure can be determined. For example, the effect of pressure on the methanol-benzene azeotrope is shown in Table I. 

When the liquid starts to boil at temperature 7 (point B), the first vapor formed has a composition yx and is therefore at its dew point, At thia point, the vapor is as rich in the light component as it will ever be. As temperature is further raised, more of the heavier component is boiled off. The quantity of vapor formed increases, but the mole fraction of the light component in both vapor and liquid drops. At temperature T2, the liquid composition is x2 and the vapor composition is y2. Some of the initial charge is now vapor and some is liquid. A further increase in temperature to Ta will vaporize the rest of the liquid. The vapor composition will now be xlt and the last drop of liquid vaporized has a composition x3, The liquid always travels along its bubble-point curve (BEH) while the vapor always travels along the dew-point curve iDFG), Therefore, in distillation, bubble-point liquid is always in equilibrium with dew-point vapor. 

FIGURE 3.10 Vapor pressure-composition curve ideal (dotted line) positive deviation (solid line) from Raoult s law. 

Figure 3.10 shows the vapor pressure/composition curve at a given temperature for an ideal solution. The three dotted straight lines represent the partial pressures of each constituent volatile liquid and the total vapor pressure. This linear relationship is derived from the mixture of two similar liquids (e.g., propane and isobutane). However, a dissimilar binary mixture will deviate from ideal behavior because the vaporization of the molecules A from the mixture is highly dependent on the interaction between the molecules A with the molecules B. If the attraction between the molecules A and B is much less than the attraction among the molecules A with each other, the A molecules will readily escape from the mixture of A and B. This results in a higher partial vapor pressure of A than expected from Raoult s law, and such a system is known to exhibit positive deviation from ideal behavior, as shown in Figure 3.10. When one constituent (i.e., A) of a binary mixture shows positive deviation from the ideal law, the other constituent must exhibit the same behavior and the whole system exhibits positive deviation from Raoult s law. If the two components of a binary mixture are extremely different [i.e., A is a polar compound (ethanol) and B is a nonpolar compound (n-hexane)], the positive deviations from ideal behavior are great. On the other hand, if the two liquids are both nonpolar (carbon tetrachloride/n-hexane), a smaller positive deviation is expected.

The viscosities of many binary liquid systems display minima as functions of composition at constant temperature, so that negative values of D are also possible. Yajnik and his coworkers (265 ) long ago observed that very frequently an extremum in the isothermal vapor pressure-composition curve is accompanied by an extremum of the opposite sense in the viscosity-concentration curve. Data are apparently not available for solutions of very low-molecular-weight paraffins in carbon tetrachloride, but minima are found for the viscosities of solutions of CC14 with ethyl iodide, ethyl acetate and acetone, so that a minimum appears quite probable for mixtures of small aliphatic hydrocarbons with carbon tetrachloride. If this were true, the downward trend of the Meyer-Van der Wyk data on C17—C31 paraffins, earlier discussed in connection with the polyethylene plots of Fig. 14, would be understood. It will be recognized that such a trend is also precisely what is to be expected from the draining effect of the hydrodynamic theories of Debye and Bueche (79), Brinkman (45 ) and Kirkwood and Riseman (139). However, the absence of such a trend in the case of polyethylene... 

The great attraction of this equation is that it contains just properties of the pure species and therefore expresses K-values as functions of T and P, independent of the compositions of the liquid and vapor phases. Moreover, and 4> can be evaluated from equations of state for the pure species or from generalized correlations. This allows K-values for light hydrocarbons to be calculated and correlated as functions of T and P. However, the method is limited for any species to subcritical temperatures, because the vapor-pressure curve terminates at the critical point. 

The third plane identified in Fig. 12.1 is the vertical one perpendicular to the composition axis and indicated by MNQRSLM. When projected on a parallel plane, the lines from several such planes present a diagram such as that shown by Fig. 12.4. This is the PT diagram lines t/C, and KC2 are vapor-pressure curves for the pure species, identified by the same letters as in Fig. 12.1. Each interior loop represents the PT behavior of saturated liquid and of saturated vapor for a mixture of fixed composition the different loops are for different compositions. Clearly, the PT relation for saturated liquid is different from that for saturated vapor of the same composition. This is in contrast with the behavior of a pure species, for which the bubble line and the dew line coincide. At points A and B in Fig. 12.4 saturated-liquid and saturated-vapor lines intersect. At such points a saturated liquid of one composition and a saturated vapor of another composition have the same T and P, and the two phases are therefore in equilibrium. The tie lines connecting the coinciding points at A and at B are perpendicular to the PT plane, as illustrated by the tie line VX in Fig. 12.1. 

Example 4.32 Column grand composite curves in methanol plant Table 4.16 describes the existing base case operations for columns 1 and 2 of the methanol plant obtained from the converged simulations using the RKS equation of state to estimate the vapor properties. The activity coefficient model, NRTL, and Henry components method are used for predicting the equilibrium and liquid properties. 

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