Vapor-phase compositions, predicting from

Perhaps the key binary from an acid gas injection point of view is H2S + COz. From the study of Knapp et al. (1982) it can be seen that the PR and SRK equations result in excellent predictions for this binary with errors in the estimated bubble point pressure less than 1.5% and errors in the vapor phase composition less than 1 mol%. 

Predicting Vapor-Phase Compositions from P-T-x Data... 

The vapor-liquid equilibrium was computed from the EOS model using the reliable and robust method of Hua et al 14-16) based on interval analysis. Their method can find the correct thermodynamically stable solution to the vapor-liquid equilibrium problem with mathematical and computational certainty. Additionally, the tangent plane distance method 17,18) was used to test the predicted liquid and vapor phase compositions for global thermodynamic phase stability. 

In Example 14.3 it is stated that use of ki2- 0.131 gives poorer prediction of vapor phase compositions than use of k 2 = 0.148. Yet it appears from Figure 14.E.3 that the curve (dashed) corresponding to ki2 = 0.131 is closer to the experimental dew point curve than that for ki2 = 0.148. Determine which k 2 gives the best values for and K2 ... 

Example 8 Calculation of Rate-Based Distillation The separation of 655 lb mol/h of a bubble-point mixture of 16 mol % toluene, 9.5 mol % methanol, 53.3 mol % styrene, and 21.2 mol % ethylbenzene is to be earned out in a 9.84-ft diameter sieve-tray column having 40 sieve trays with 2-inch high weirs and on 24-inch tray spacing. The column is equipped with a total condenser and a partial reboiler. The feed wiU enter the column on the 21st tray from the top, where the column pressure will be 93 kPa, The bottom-tray pressure is 101 kPa and the top-tray pressure is 86 kPa. The distillate rate wiU be set at 167 lb mol/h in an attempt to obtain a sharp separation between toluene-methanol, which will tend to accumulate in the distillate, and styrene and ethylbenzene. A reflux ratio of 4.8 wiU be used. Plug flow of vapor and complete mixing of liquid wiU be assumed on each tray. K values will be computed from the UNIFAC activity-coefficient method and the Chan-Fair correlation will be used to estimate mass-transfer coefficients. Predict, with a rate-based model, the separation that will be achieved and back-calciilate from the computed tray compositions, the component vapor-phase Miirphree-tray efficiencies. 

Thus, the composition of the vapor phase at 1 atm is y = 0.7863, y2 = 0.2136 from the Wilson Equation. For this mixture, at these conditions, there is not much difference between the predictions of Raoult s Law and the Wilson equation, indicating only moderate deviations from ideality at the chosen conditions. 

Figure 3. Comparison of predicted and experimental N2-H2 mixture vapor phase water concentrations (system composition 75% HJ25% N2 (molar) from Ref. 3—(O) 50°C (A) 37.5°C (V) 25°C predicted—(-------------) SRK (-------) PFGC-...

Figure 3. Experimental and predicted vapor and aqueous liquid phase compositions for the carbon dioxide-water system ((-) P-R prediction data from Ref.

A critical requirement for the success of receptor models for atmospheric particles Is that the compositions of particles from all major sources in an area be accurately known. Chemical element balances (CEBs) of 130 samples taken In Washington, D.C. and analyzed for 40 elements yielded nearly the same source strengths when 28 elements are used In the least-squares fit as when only nine carefully chosen elements are used. Certain elements are important to the stability of CEB fits (Na, Ca, V, Mn, As and Pb) and should be measured carefully In particles from sources. For three of the nine elements (Al, Fe and Zn), other elements can serve as surrogates (many lithophlles for Al and Fe, Sb and Cd for Zn). Measurements on many more sources of each Important type should be done In order that trends can be observed that will allow one to predict compositions of particles from unmeasured sources. Instack measurements should Include collections of at least two size fractions of particles plus vapor-phase species. Measurements of at least 20 elements plus some classes of carbonaceous material should be made. 

A review is presented of techniques for the correlation and prediction of vapor-liquid equilibrium data in systems consisting of two volatile components and a salt dissolved in the liquid phase, and for the testing of such data for thermodynamic consistency. The complex interactions comprising salt effect in systems which in effect consist of a concentrated electrolyte in a mixed solvent composed of two liquid components, one or both of which may be polar, are discussed. The difficulties inherent in their characterization and quantitative treatment are described. Attempts to correlate, predict, and test data for thermodynamic consistency in such systems are reviewed under the following headings correlation at fixed liquid composition, extension to entire liquid composition range, prediction from pure-component properties, use of correlations based on the Gibbs-Duhem equation, and the recent special binary approach. 

Thermodynamic predictions of the solid-phase composition have been very successful for the growth by MOCVD of group III-V compound semiconductors (e.g., InAs Sb and GaAs SbJ even though the gas-phase reactions are far from equilibrium (88-91). The procedure is also useful for estimating solid-vapor distribution coefficients of group II-VI compound semiconductors (e.g., Cd Hg e and ZnSe SJ grown by MOCVD (92). In the analysis, the gas phase is considered to be an ideal mixture, that is... 

For these data, assume the vapor phase an ideal gas and plot P vs. x, P vs. yu y,P vs. x, and y2 P vs. x,. Determine Henry s constant for each species from the partial-pressure curves. For each species, over what composition range does Henry s law predict partial pressures within 5 percent of the true values ... 

In these equations the vapor compositions, vb and yc. and the equilibrium pressure P are unknown (the equilibrium pressure is 1.013 bar only at xb — 0.525). The solution is obtained by choosing a value of xg, using xc = 1 — xb, and copiputing ys and yc from Eqs. i, and the total pressure from Eq. iii. The vapor-phase mole fractions are then computed from Eqs. ii. The results of this calculation are given in the table and Fig. 2. Regular solution model. Since benzene and cyclohexane are nonpolar, and their solubility parameters are given in Table 9.6-1, the activity coefficients can be predicted using Eqs. 

It is clear that the (fifference in volatility of the various components of a liquid mixture is a to the successfid qiplkation of distillation. This difference can be related to the thermodynamic equilibrium that can exist between die liquid and vqxir mixtures under conditions that can be associated with the distillation at hand. The phase equiUbrUon relationships are embodied in the general area of solution thermodynamics and can be measured or, in some cases, predicted from the properties of the pure materials involved. The restthiiig equiiibiium compositions often are referred to as vapor-lUpud e dUbriwn data, shortened to vapor-Uquid equilibria and aHneviated simply as VLB. There are occaaonal instances when a second immiscible liquid phase is involved, with compositions of the three phases at diermodynamic equilibrium known simply as vapm--U[Pg.230]

Starting from either side of the phase diagram, the situation is very much like a liquid-vapor phase change One component will preferentially change phase, and the other component will become more and more concentrated within the remaining liquid. Until, that is, a certain composition labeled is reached Then the two components will freeze simultaneously, and the solid that forms will have the same composition as the liquid. This composition is called the eutectic composition. At this composition, this liquid acts as if it were a pure component, so the solid and liquid phases have the same composition when in equilibrium at the eutectic temperature Tg. This pure component is called the eutectic. The eutectic is similar to the azeotrope in liquid-vapor phase diagrams. Not all systems will have eutectics some systems may have more than one, and the composition of the eutectic(s) of a multicomponent system is characteristic of the components. That is, you cannot predict a eutectic for any given system. 

The necessity for reliable VLE data is apparent. Many separations processes involve the transfer of chemical species between contiguous liquid and vapor phases. Rational design and simulation of these processes requires knowledge of the equilibrium compositions of the phases. Raoult s and Henry s "laws" rarely suffice as quantitative tools for the prediction of equilibrium compositions precise work demands the availability of either the equilibrium data themselves, or of thermodynamic correlations derived from such data. 

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