Heterogeneous liquid-vapor system

This focus on phase behavior brought about the understanding that some of the reactions that had been successfully accomplished in supercritical carbon dioxide were actually taking place in the liquid phase (or at the interface) of a heterogeneous liquid-vapor system. Similarly, reactions carried out in liquid, near-critical propane had shown that this solvent displayed the same advantages as those of truly supercritical ones. The fact is that the densities of liquid mixtures close to the mixture s critical line are sufficiently lower than those of a classical Hquid to exhibit the same type of property values (lower viscosity, higher diffusivity, higher so-... 

Modeling of a semibatch reactor (Figure 16.1) enables to determine the reaction rate pseudoconstants. For lack of physical data, a number of assumptions have to be made. The volume of the liquid phase is the function of composition, temperature, pressure, and mass of EO reacted with raw material. At a constant temperature (185 5°C), the volume of the liquid phase increases due to an increased solubility of EO. However, the rate of change is relatively low compared to the reaction rate. The universal functional activity coefficient (UNIFAC) method [43] was used to calculate the activity coefficients. The method was adopted for the heterogeneous liquid-liquid-vapor system as the limited solubility of liquid components was observed. The... 

Radiation-induced grafting in its simplest form involves heterogeneous systems, with the substrate being film, fiber or even powder, and with the monomer to be grafted onto the substrate being a neat liquid, vapor or solution.231-236 Currently, three main radiation grafting techniques are known.232 237... 

For a ternary system the composition of vapor in equilibrium with a heterogeneous liquid mixture can be related to the liquid compositions by two relationships (5). First, the same vapor composition will result from any liquid composition on a given liquid-liquid tieline. Second, a... 

At negative pressure, these scenarios differ with respect to the shapes of the LDM and of the liquid-vapor spinodal curve Ps T) scenario (i) predicts a monotonic LDM and a minimum of Ps as a function of temperature, whereas scenarios (ii) and (iii) predict a turning point in the LDM and a monotonic spinodal. In an experiment, it is difficult to reach the spinodal rather, the liquid will break before by nucleation of vapor bubbles (cavitation). Usually, impurities favor heterogeneous nucleation, and lead to irreproducible results. But for a pristine system, nucleation will occur homogeneously, at a well-defined pressure threshold / cav(T), which is an intrinsic property of the liquid. 

The Polanyi-Manes model is postulated to follow a pore-filling mechanism, which was first applied by Xia and Ball [35], and later applied by other research groups [34, 36] to describe sorption of several HOCs by selected natural soils and sediments. The Polanyi adsorption model originally was set up for the quantification of the adsorption of gas molecules to energetically heterogeneous solids, and was extended to a wide range of vapor and liquid phase systems by Manes and his co-workers. The Polanyi theory considers that, for a molecule located within the attractive force field of a micro-porous solid, there exists an... 

Fig. 15. Isobaric vapor—liquid—liquid (VLLE) phase diagrams for the ethanol—water—benzene system at 101.3 kPa (D-D) representHquid—Hquid tie-lines (A—A), the vapor line I, homogeneous azeotropes , heterogeneous azeotropes Horsley s azeotropes, (a) Calculated, where A is the end poiat of the vapor line and the numbers correspond to boiling temperatures ia °C of 1, 70.50 2, 68.55 3, 67.46 4, 66.88 5, 66.59 6, 66.46 7, 66.47, and 8, the critical poiat, 66.48. (b) Experimental, where A is the critical poiat at 64.90°C and the numbers correspond to boiling temperatures ia °C of 1, 67 2, 65.5 3, 65.0 ...

An example of heterogeneous-azeotrope formation is shown in Fig. 13-13 for the water-normal butanol system at 101.3 kPa. At liquid compositions between 0 and 3 mole percent butanol and between 40 and 100 mole percent butanol, the liquid phase is homogeneous. Phase sphtting into two separate liquid phases (one with 3 mole percent butanol and the other with 40 mole percent butanol) occurs for any overall hquid composition between 3 and 40 mole percent butanol. A miuimum-boihug heterogeneous azeotrope occurs at 92°C (198°F) when the vapor composition and the over l composition of the two liquid phases are 75 mole percent butanol. 

Three types of binary equilibrium cui ves are shown in Fig. 13-27. The y-x diagram is almost always plotted for the component that is the more volatile (denoted by the subscript 1) in the region where distillation is to take place. Cui ve A shows the most usual case, in which component 1 remains more volatile over the entire composition range. Cui ve B is typical of many systems (ethanol-water, for example) in which the component that is more volatile at lowvalues of X becomes less volatile than the other component at high values of X. The vapor and liquid compositions are identical for the homogeneous azeotrope where cui ve B crosses the 45° diagonal. A heterogeneous azeotrope is formed with two liquid phases by cui ve C,... 

FIG. 13-27 Typical binary eqiiilihriiim curves. Curve A, system with normal volatility. Curve B, system with homogeneous azeotrope (one liquid phase). Curve C, system with heterogeneous azeotrope (two liquid phases in eqiiilih-riiim with one vapor phase). 

Figure 8-7. System with heterogeneous azeotrope-two liquid phases in the equilibrium with one vapor phase. Used by permission. Smith, B.D., Design of Equilibrium Stage Processes, McGraw-Hiii, New York (1963), all rights reserved.

Chemical equilibria with reactants and products that are all in the same phase are called homogeneous equilibria. Equilibria C, D, and E are homogeneous. Equilibria in systems having more than one phase are called heterogeneous equilibria. Equilibrium F is heterogeneous so too is the equilibrium between water vapor and liquid water in a closed system ... 

Many semibatch reactions involve more than one phase and are thus classified as heterogeneous. Examples are aerobic fermentations, where oxygen is supplied continuously to a liquid substrate, and chemical vapor deposition reactors, where gaseous reactants are supplied continuously to a solid substrate. Typically, the overall reaction rate wiU be limited by the rate of interphase mass transfer. Such systems are treated using the methods of Chapters 10 and 11. Occasionally, the reaction will be kinetically limited so that the transferred component saturates the reaction phase. The system can then be treated as a batch reaction, with the concentration of the transferred component being dictated by its solubility. The early stages of a batch fermentation will behave in this fashion, but will shift to a mass transfer limitation as the cell mass and thus the oxygen demand increase. 

The scale of components in complex condensed matter often results in structures having a high surface-area-to-volume ratio. In these systems, interfacial effects can be very important. The interfaces between vapor and condensed phases and between two condensed phases have been well studied over the past four decades. These studies have contributed to technologies from electronic materials and devices, to corrosion passivation, to heterogeneous catalysis. In recent years, the focus has broadened to include the interfaces between vapors, liquids, or solids and self-assembled structures of organic, biological, and polymeric nature. 

Industrial problems have, in some instances, been solved either by a proper choice of construction materials and suitable process design or by development of heterogeneous catalytic systems using supported complexes or by generating active complexes in situ on a support material which avoid some of the problems of liquid-phase operation. For example, a number of the problems in liquid-phase vinyl acetate processing have been overcome by development of supported Pd catalysts (106). Vapor-phase hydroformylation has been carried out on supported rhodium complexes (107). 

In cases where one of two resultants can be separated from the reactants and the other resultant, by precipitation as a solid, by condensation as a liquid, or by volatilization as a gas or vapor, the yield of the desired substance from a given amount uf reactants can sometimes he materially increased. In the ease of heterogeneous systems, whenever a solid participant is present, the t tmt eniwlitm of said solid is considered constant. The precipitation and solution or solids are in this category, as well as the reactions between a gas and a solid, e.g.. the system ferrule nic oxide plus hydrogen gas plus iron plus water vapor. 

The two-film model representation can serve as a basis for more complicated models used to describe heterogeneously catalyzed RSPs or systems containing suspended solids. In these processes a third solid phase is present, and thus the two-film model is combined with the description of this third phase. This can be done using different levels of model complexity, from quasi-homogeneous description up to the four-film presentations that provide a very detailed description of both vapor/gas/liquid-liquid and solid/liquid interfaces (see, e.g., Refs. 62, 68 and 91). A comparative study of the modeling complexity is given in Ref. 64 for fuel ether synthesis of MTBE and TAME by CD. 

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