Phase changes liquid-vapour

The term sublimation strictly refers to the phase change solid -> vapour, with no intervention of a liquid phase. In industrial applications, however, the term usually includes the reverse process of condensation or desublimation solid -> vapour -> solid. In practice, it is sometimes desirable to vaporise a substance from the liquid state and hence the... 

Adding the mass balance equations for water and air to the equation for energy balance and momentum balance (equilibrium), the governing equations for the THM problem are obtained. Theses equations are solved simultaneously at every time step of the analysis. To close the THM formulation it is necessary to specify the relevant constitutive equations that describe the set of phenomena considered. Phase changes (liquid water/vapour and gaseous air/ dissolved air) are assumed to be in equilibrium. The corresponding equilibrium restrictions are also incorporated in the formulation. More details are given in Olivella et al. (1994)... 

So far, in the discussion of industrial crystallization processes, only the crystallization of a solid phase from a supersaturated or supercooled liquid phase has been considered. However, the crystallization of a solid substance can be induced from a supersaturated vapour by the process generally known as sublimation . Strictly speaking, of course, the term sublimation refers only to the phase change solid vapour without the intervention of the liquid phase. In its industrial application, however, the term is commonly used to include the condensation (crystallization) process as well, i.e. solid vapour... 

The trends in Figure Sa can be understood by referring to the phase behaviour summarized in Figure 3. At 260 K, the L2-G binodal approaches a critical point as pressure rises and thus the interfacial tension drops. The critical point is never reached because the L2-G binodal is truncated by the separation of a second liquid phase. At the three-phase equilibrium point there are three interfocial tensions, namely those of the Ll-G, L2-G, and L1-L2 interfaces, while the interfacial tension between the LI and G phases differs from that between the L2 and G phases. The liquid-vapour interfocial tension changes discontinuously with pressure as the three-... 

Carey VP (1992) Liquid-vapour phase-change phenomena. Hemisphere, Washington, DC Collier SP (1981) Convective boiling and condensation. McGraw-Hill, New York Ha JM, Peterson GP (1998) Capillary performance of evaporation flow in micro grooves an analytical approach for very small tilt angles. ASME J Heat Transfer 120 452 57 Hetsroni G, Yarin LP, Pogrebnyak E (2004) Onset of flow instability in a heated capillary tube. Int J Multiphase Flow 30 1424-1449... 

The variation of enthalpy for binary mixtures is conveniently represented on a diagram. An example is shown in Figure 3.3. The diagram shows the enthalpy of mixtures of ammonia and water versus concentration with pressure and temperature as parameters. It covers the phase changes from solid to liquid to vapour, and the enthalpy values given include the latent heats for the phase transitions. 

The precise location of the feed point will affect the number of stages required for a specified separation and the subsequent operation of the column. As a general rule, the feed should enter the column at the point that gives the best match between the feed composition (vapour and liquid if two phases) and the vapour and liquid streams in the column. In practice, it is wise to provide two or three feed-point nozzles located round the predicted feed point to allow for uncertainties in the design calculations and data, and possible changes in the feed composition after start-up. 

Figure 5.6 Freeze-drying works by decreasing the pressure, and causing a phase change at higher pressure, the stable form of water is liquid, but the stable form at lower pressures is vapour. Consequently, water (as vapour) leaves a sample when placed in a vacuum or low-pressure chamber we say the sample is freeze-dried ...

Let us first consider how the density of the condensed phase changes with temperature. As our material in the vapour phase is cooled at constant pressure the density increases until the boiling point is reached. Further cooling then allows us to differentiate between the vapour and liquid states by the formation of a boundary. Further cooling increases the liquid density but at a much slower rate than that of the gaseous phase. The density of many liquids can be described by a simple linear equation over a wide temperature range 5... 

The explosive phenomena produced by contact of liquefied gases with water were studied. Chlorodifluoromethane produced explosions when the liquid-water temperature differential exceeded 92°C, and propene did so at differentials of 96-109°C. Liquid propane did, but ethylene did not, produce explosions under the conditions studied [1], The previous literature on superheated vapour explosions has been critically reviewed, and new experimental work shows the phenomenon to be more widespread than had been thought previously. The explosions may be quite violent, and mixtures of liquefied gases may produce overpressures above 7 bar [2], Alternative explanations involve detonation driven by phase changes [3,4] and do not involve chemical reactions. Explosive phase transitions from superheated liquid to vapour have also been induced in chlorodifluoromethane by 1.0 J pulsed ruby laser irradiation. Metastable superheated states (of 25°C) achieved lasted some 50 ms, the expected detonation pressure being 4-5 bar [5], See LIQUEFIED NATURAL GAS, SUPERHEATED LIQUIDS, VAPOUR EXPLOSIONS... 

As air diffuses through the samples during the controlled stripping in the test, the concentration of the more volatile components of the liquid phase changes with time as the stripping continues. The presence of two volumes of liquid in series is sufficient for most components to prevent alteration of the vapour/liquid equilibrium, but for some very volatile substances, the concentration in the exit air will inevitably change with time. 

Note that the mass flux of component i in the liquid phase changes due to chemical conversion, whereas this flux remains constant in the vapour/gas phase since it has been assumed that no reaction occurs in the vapour/gas phase. For both phases the conservation for thermal energy equation is given by... 

There is a distinct region of small aggregates or clusters which falls between the atomic (or molecular) domain and that of condensed matter. These small particles and clusters possess unique properties and have several technological applications. The formation of these particles involves a vapour-solid, a liquid-solid, a solid-solid or a vapour-liquid-solid type of phase change governed by nucleation and it is important that the size of the growing nucleus is controlled (Multani, 1981 Hadjipanyas Siegel, 1994). 

Our further analysis of the enthalpy of formation of 2-methylbicyclo[2.2.1]heptene only worsens the disparity. That is, we find methylation of one doubly bonded carbon in gaseous cyclopropene, cyclopentene and cyclohexene is accompanied by a decrease in enthalpy of formation of 34, 38 and 38 kJmol-1, i.e. 36 2 kJmol-1. The recommended enthalpy of formation of bicyclo[2.2.1]heptene (see Reference 60) is 90 kJmol-1 and so we would predict an enthalpy of formation of its gaseous 2-methyl derivative of 90 — 36 54 kJmol-1. Using our standard enthalpy of vapourization estimation protocol we would predict a phase-change enthalpy of 40 kJmol-1 for this species, and so derive an enthalpy of formation of liquid 2-methylbicyclo[2.2.1]heptene of ca 54-40 15 kJmol-1. That is, if anything, the exocyclic species is too stable if we compare this derived value with 4.5 1.8 kJmol-1 derived from the available combustion calorimetric data. 

Let s first define exactly what we are talking about Two-phase flow describes a condition whereby a flow stream contains fluid in the liquid phase and at the same time in the gas or vapour phase. Flashing flow occurs as a result of a decrease in pressure, and all or a portion of a liquid flow changes into vapour. It is possible for both flowing conditions, two-phase and flashing, to occur simultaneously within the same application. The complexity of the issue results from the fact that this condition is never stable and constantly changes during a relief cycle. 

Colloidal dispersions can be formed either by nucleation with subsequent growth or by subdivision processes [12,13,16,25,152,426], The nucleation process requires a phase change, such as condensation of vapour to yield liquid or solid, or precipitation from solution. Tadros reviews nucleation/condensation processes and their control [236], Some mechanisms of such colloid formation are listed in Table 7.1. The subdivision process refers to the comminution of particles, droplets, or bubbles into smaller sizes. This process requires the application of shear. Some of the kinds of devices used are listed in Table 7.2 [228]. 

For latent heat, we look up the corresponding entry in the tables for either the latent heat of vapourisation (or simply the heat of vapourisation) or the heat of fusion, depending on the type of phase change encountered (liquid to vapour and solid to liquid, respectively). These quantities are in units of energy per unit mass and are given for a specific reference state (often the 1 atm boiling point or melting point of the substance). 

CHEMICAL - involving reaction and chemical change or PHYSICAL - involving physical change (e.g. variation of pressure, P, or phase such as in boiling (liquid - vapour). 

The process of boiling is defined as the phase change taking place when a liquid is converted into a vapour or gaseous state). 

This equation, evidently, gives change in pressure dP which must accompany the change in temperature dT or vice versa, in the case of a system containing two phases of a pure substance in equilibrium with each other. Suppose the system consists of water in the two phases, viz., liquid and vapour, in equilibrium with each other at the temperature T, i.e.,... 

In the real polydisperse foam along with coalescence there always acts another process of internal collapse. This is the diffusion decrease in the specific surface which is accompanied by structural rearrangement, i.e. shift of knots and borders, and change in their orientation. This leads to the origination of various local disturbances (Act, Apa, AC, etc.). These local disturbances along with the rupture of individual films cause destruction either of other films and borders or of local volumes or of the whole foam (see Sections 6.5 and 6.6). Finally, various external factors can affect the foam (pressure drop, applied to the liquid phase reduced pressure of the liquid vapour above the foam, leading to evaporation the effect of antifoam droplets a-particle irradiation vibration, etc.). 

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