VLLE Behavior

Number Or Components 3 Heterogeneous Temperature 337.17 K Classification Unstable Node  

Now that the complexity of the VLLE is apparent, let us develop a simulation of a flowsheet to produce high-purity ethanol and water. The flowsheet will have two distillation columns and a decanter. There are two recycle streams back to the first column organic phase from the decanter and distillate from the second column. 

One way to estimate these compositions is to recognize that the overhead vapor from the first column will have a composition that is close to the ternary azeotrope 53.06/27.49/19.45 mol% (B/E/W). We set up a simulation with a stream with this composition feeding a decanter operating at 313 K. The predicted compositions of the organic and aqueous liquid phases are 

STREAMS DSTWU Dirt Ra iFtac Eaiaa MutaFim SCTmc 

The composition of the organic reflux should be close to the composition of the organic liquid phase. The composition of the feed to the second column should be close to the composition of the aqueous liquid phase. As essentially all the water in the feed comes out at the bottom of the second column at a high purity, the amount of water removed from the feed is only (0.06 kmol/s)(0.16) = 0.0096 kmol of water/s. Therefore, as a first estimate, we can use the composition of the aqueous liquid phase for a guess of the recycle composition. 

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In Part 3 of this book an extrainer is added to the system so that liquid-liquid sphtting can appear in the top decanter and also maybe in the top few stages of the azeotropic column. The LLE behavior in the decanter, or the VLLE behavior in the top stages of flie azeotropic column, can be predicted by Aspen Plus. The system of separating an isopropanol-water mixture using cyclohexane as the entrainer will be used as an example to demonstrate the way to generate a LLE envelope in Aspen Plus. 

Category IV phase behavior, shown in Fig. 10.3-3c/, has two regions of liquid-liquid-vapor equilibrium. The low-temperature VLLE region exhibits category II behavior, whereas the higher-temperature VLLE region behaves like a category V system. 

Figure 11.3-1 Predicted phase behavior of the isobutane-furfural mixture at 37.8°C. Note that. regions of a single vapor (V), single liquid (Li, Li), vapor-liquid (VLE), liquid-liquid (LLE), and vapor-liquid-liquid (VLLE) are predicted to occur.

We now describe the phase behavior exhibited by binary mixtures at modest pressures. The kinds of behavior observed in Nature include vapor-liquid equilibria (VLE, 9.3.1-9.3.3), azeotropes ( 9.3.4), critical points ( 9.3.5), liquid-liquid equilibria (LLE, 9.3.6), and vapor-liquid-liquid equilibria (VLLE, 9.3.7). When solid-fluid equilibria occur ( 9.4), many (but not all) of the resulting phase diagrams are analogous to their counterparts in fluid-fluid equilibria for example, many liquid-solid diagrams are analogous to vapor-liquid diagrams. 

When the mixture critical line is discontinuous, it is divided into two branches by a three-phase VLLE line, as shown in Figure 9.22. The high temperature end of the VLLE line is a UCEP that connects one branch of the critical line to the critical point of the more volatile component. This branch of the critical line is usually short. Based on the behavior of the critical branch emanating from the critical point of the less volatile component, we divide these mixtures into two classes. 

For the design study of a particular separation system, we typically start by using the Aspen built-in parameters of a suitable physical property model. The phase equilibrium behavior predicted by the Aspen built-in parameters should be compared with experimental data for validation purpose. It is obvious that an inaccurate description of the phase equilibrium behavior of a separation system will give flowsheet results that do not match the results of the true system. The worst case may be a failure of the separation task in the proposed design flowsheet. Thus, the validation stage is important before doing any design study. The experimental data that can typically be found in hterature include the Txy and yx data, binary and ternary LLE data, VLLE data, and azeotropic information. 

Note that selecting a proper physical property method is extremely important for obtaining reliable simulation results of a process flowsheet. Aspen Physical Property System gives the recommended classes of property methods for different applications. The phase equilibrium behavior predicted by the selected physical property method should be compared to experimental data for validation purpose. The experimental data that can typically be found in literamre includes the Txy and yx data binary and ternary LLE data VLLE data and azeotropic information. For details of how to validate the prediction of the selected physical property method, please refer to Chapter 2. 

Fig. 10 Phase behavior of the system PMMA (M = 18 kg/mol)/MMAA 02 at 65°C [45]. Squares and circles represent experimental cloud-point data. The solid lines (phase boundary) and dashed lines (tie lines) are calculated with PC-SAFT. Triangles represent the VLLE region, (a) p = 100 bar, (b) p = 150 bar...

Construct phase diagrams for binary systems in vapor-liquid equilibria (VLE), liquid-liquid equilibria (LLE), vapor-liquid-liquid equilibria (VLLE), solid-liquid equilibria (SLE), solid-solid equilibria (SSE), and solid-solid-liquid equilibria (SSLE), correcting for nonideal behavior in the vapor, liquid, or solid phases using fugacity coefficients and activity coefficients. 

Treat the solubility of gases in liquids using Henry s law for both ideal and nonideal behavior. Correct reported Henry s law coefficients for pressure or temperature. Perform LLE, VLLE, SLE, and SSE phase equilibria calculations. Determine whether a liquid mixture is inherently instable and will split into two liquid phases. 

In this section, we consider the case when three phases are in equilibrium a vapor phase and two liquid phases, a and p. A generic diagram of a system with m components in vapor-liquid-liquid (VLLE) equilibrium is shown in Figure 8.15. How does such behavior come about Let s return to the binary mixture of a and b. Consider the case where we have both an azeotrope in VLE and liquid-liquid equilibrium (ELE). This scenario corresponds to a minimum-boiling azeotrope where the like interactions are stronger than the unlike interactions. Figure 8.16a shows the phase diagram for the case... 

The lowest possible temperature in which we have only liquid is called the eutectic point. The eutectic point, marked in Figure 8.17a, is the point at which the equilibrium line of liquid for the solid b-liquid binary intersects with that of the solid a-liquid binary. At the eutectic temperature, we have SSLE where three phases can exist in equilibrium (solid a, solid b, and liquid) much like the behavior of VLLE in Section 8.3. At this point the binary system is completely constrained, so aU the properties are fixed. 

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