Vapor-liquid equilibrium phase equilibria, high pressure

Pervaporation. Pervaporation differs from the other membrane processes described so far in that the phase-state on one side of the membrane is different from that on the other side. The term pervaporation is a combination of the words permselective and evaporation. The feed to the membrane module is a mixture (e.g. ethanol-water mixture) at a pressure high enough to maintain it in the liquid phase. The liquid mixture is contacted with a dense membrane. The other side of the membrane is maintained at a pressure at or below the dew point of the permeate, thus maintaining it in the vapor phase. The permeate side is often held under vacuum conditions. Pervaporation is potentially useful when separating mixtures that form azeotropes (e.g. ethanol-water mixture). One of the ways to change the vapor-liquid equilibrium to overcome azeotropic behavior is to place a membrane between the vapor and liquid phases. Temperatures are restricted to below 100°C, and as with other liquid membrane processes, feed pretreatment and membrane cleaning are necessary. 

Ohgaki, K. and Katayama, T. 1977. "Isothermal Vapor-Liquid Equilibrium Data for the Ethane-Carbon Dioxide System at High Pressure" Fluid Phase Equil., 1 27-32. 

A modified local composition (LC) expression is suggested, which accounts for the recent finding that the LC in an ideal binary mixture should be equal to the bulk composition only when the molar volumes of the two pure components are equal. However, the expressions available in the literature for the LCs in binary mixtures do not satisfy this requirement. Some LCs are examined including the popular LC-based NRTL model, to show how the above inconsistency can be eliminated. Further, the emphasis is on the modified NRTL model. The newly derived activity coefficient expressions have three adjustable parameters as the NRTL equations do, but contain, in addition, the ratio of the molar volumes of the pure components, a quantity that is usually available. The correlation capability of the modified activity coefficients was compared to the traditional NRTL equations for 42 vapor—liquid equilibrium data sets from two different kinds of binary mixtures (i) highly nonideal alcohol/water mixtures (33 sets), and (ii) mixtures formed of weakly interacting components, such as benzene, hexafiuorobenzene, toluene, and cyclohexane (9 sets). The new equations provided better performances in correlating the vapor pressure than the NRTL for 36 data sets, less well for 4 data sets, and equal performances for 2 data sets. Similar modifications can be applied to any phase equilibrium model based on the LC concept. 

At pressures above a few atmospheres, the deviations from ideal behavior in the gas phase will be significant and must be taken into account in process design. The effect of pressure on the liquid-phase activity coefficient must also be considered. A discussion of the methods used to correlate and estimate vapor-liquid equilibrium data at high pressures is beyond the scope of this book. Refer to the texts by Null (1970), Prausnitz et al. (1998), or Prausnitz and Chueh (1968). 

We now turn from the qualitative description of high-pressure phase equilibria and its measurement to the quantitative description, that is, to the correlation or prediction of vapor-liquid equilibrium for hydrocarbon (and light gas). systems, of which the ethane-propylene system is merely one example. Our interest will be only in systems describable by a single equation of state for both the vapor and liquid phases, as the case in which the liquid is described by an activity coefficient model was considered in the previous section. 

Bertucco et al. investigated the effect of SCCO2 on the hydrogenation of unsaturated ketones catalyzed by a supported Pd catalyst, by using a modified intemal-recycle Berty-type reactor [63]. A kinetic model was developed to interpret the experimental results. To apply this model to the multiphase reaction system, the calculation of high-pressure phase equilibria was required. A Peng-Robinson equation of state with mixture parameters tuned by experimental binary data provided a satisfactory interpretation of all binary and ternary vapor-liquid equilibrium data available and was extended to multicomponent... 

To date little or no thermodynamic modeling of the phase behavior of the ligand/C02 or metal chelate/C02 systems has been conducted. However, in order for supercritical fluid extraction to be considered as a possible replacement for organic solvent extraction, accurate models must be developed to predict the phase behavior of these systems to allow for both equipment and process design. Equation of state (EOS) modeling was chosen here to model the vapor-liquid equilibrium of the P-diketone/C02 systems studied. Cubic EOSs are the most widely used in modeling high pressure and supercritical fluid systems. This is... 

The new supplementary volume is again divided into the seven chapters as used before (1) Introduction, (2) Vapor-Liquid Equilibrium (VLE) Data and Gas Solubilities of Copolymer Solutions, (3) Liquid-Liquid Equilibrium (LLE) Data of Copolymer Solutions, (4) High-Pressure Fluid Phase Equilibrium (HPPE) Data of Copolymer Solutions, (5) Enthalpy Changes for Copolymer Solutions, (6) PVT Data of Copolymers and Solutions, and (7) Second Virial Coefficients (A2) of Copolymer Solutions. Finally, appendices quickly route the user to the desired datasets. 

There are about 850 newly published referenees containing about 150 new vapor-liquid equilibrium data sets and some new tables containing classical Henry s coefficients, about 600 new liquid-liquid equilibrium data sets and some new high-pressure fluid phase equilibrium data, 10 new enthalpic data sets, 20 new data sets describing PVT-properties of polymers, and 120 new data sets with densities or excess volumes. There are also new results on second osmotic virial coefficients of about 45 polymers in aqueous solution. So, in comparison to the original handbook, the new supplementary volume contains even a larger amoimt of data and will be a useful as well as necessary completion of the origirral handbook. 

Whereas in the vapor-liquid equilibrium calculation at low pressures, the model is only used to correct for the nonideality of the liquid phase, the results for liquid-liquid equilibrium calculations and even whether a demixing is calculated or not, is completely determined by the model used. Therefore, liquid-liquid calculations are always much more challenging for any model. Figure 7 demonstrates the ability of PC-SAFT to model also liquid-liquid demixing in polymer systems with high accuracy. The calculations are performed for the system pol5q)ropylene/ -pentane at three different temperatures. Although only temperature-independent pure-component and binary parameters were used, the experimental data can be described very well and the model even correctly captures the temperature dependence of the miscibility gap. 

This vessel separates toluene and benzene as a liquid from the noncondensable gases hydrogen and methane. The reactor product is cooled and forms a vapor and a liquid stream that are in equilibrium. The vapor-liquid equilibrium is that at the temperature and pressure of the stream entering V-102. From Tables 6.1 and 62, we conclude that the lower tenperature (38°C) was provided to obtain a liquid phase for the vapor-liquid equilibrium The pressure was maintained to support the formation of the liquid phase. Because the separation can be affected relatively easily at high pressure, it is worthwhile maintaining V-102 at this high pressure. 

Ohgaki, K. Katayama, T. Isothermal vapor-liquid equilibrium data for the ethane - earbon dioxide system at high pressures. Fluid Phase Equilib. 1977, 1, 27-32. 

We will concentrate on Vapor-Liquid Equilibrium (VLE) because of its wide applicability. In addition, the methodology involved and discussed here is typical to all types of phase equilibria. Finally, for convenience reasons that will become apparent in Section 13.4, we consider in this Chapter Low Pressure VLE and High Pressure VLE, in the next one. 

In practice vapor-liquid equilibrium calculations using an equation of state, that is applicable to both phases, can be carried out with sufficient accuracy only for systems that are up to moderately nonideal, typically hydrocarbon mixtures alone or with such gases as H2S, CO, CO2, etc. Furthermore, since separation of such systems by distillation - because of their typically low boiling point temperatures at atmospheric pressure - is carried out at high pressures, this methodology represents what is traditionally referred to as the High Pressure or Equation of State approach to... 

Foot et al. [22] have determined experimentally the high-pressure phase behavior of the binary systems (butane adamantane) and (butane -I- diamantane). The phase behavior of these binary systems is shown schematically in Figure 1.7. Because the phase diagrams of pure adamantane and diamantane show a solid-solid (si + S2) transition line the curve representing the (solid diamondoids -I- liquid + vapor) equilibrium will split into two branches. One branch corresponds to the (si -f 1 -f v) equilibrium and the other branch corresponds to the (. 2 1 ) equilibrium. Both branches intersect at the (si S2) equilibrium line of the pure diamondoids. The... 

In vapor-liquid equilibria, it is relatively easy to start the iteration because assumption of ideal behavior (Raoult s law) provides a reasonable zeroth approximation. By contrast, there is no obvious corresponding method to start the iteration calculation for liquid-liquid equilibria. Further, when two liquid phases are present, we must calculate for each component activity coefficients in two phases since these are often strongly nonlinear functions of compositions, liquid-liquid equilibrium calculations are highly sensitive to small changes in composition. In vapor-liquid equilibria at modest pressures, this sensitivity is lower because vapor-phase fugacity coefficients are usually close to unity and only weak functions of composition. For liquid-liquid equilibria, it is therefore more difficult to construct a numerical iteration procedure that converges both rapidly and consistently. 

The vapor pressure of a crude oil at the wellhead can reach 20 bar. If it were necessary to store and transport it under these conditions, heavy walled equipment would be required. For that, the pressure is reduced (< 1 bar) by separating the high vapor pressure components using a series of pressure reductions (from one to four flash stages) in equipment called separators , which are in fact simple vessels that allow the separation of the two liquid and vapor phases formed downstream of the pressure reduction point. The different components distribute themselves in the two phases in accordance with equilibrium relationships. 

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