Evaporation and Condensation of Liquids

Molecules of liquid in the gas phase, such as water vapor molecules above a body of water, may come together or strike the surface of the liquid and reenter the liquid phase. This process is called condensation. It is what happens, for example, when water vapor molecules in the atmosphere strike the surface of very small water droplets in clouds and enter the liquid water phase, causing the droplets to grow to sufficient size to faU as precipitation. 

Vapor pressure is very important in determining the fates and effects of substances, including hazardous substances, in the environment. Loss of a liquid by evaporation to the atmosphere increases with increasing vapor pressure and is an important mechanism by which volatile pollutants enter the atmosphere. High vapor pressure of a flammable liquid can result in the formation of explosive mixtures of vapor above a liquid, such as in a storage tank. 

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Transient cavitation is generally due to gaseous or vapor filled cavities, which are believed to be produced at ultrasonic intensity greater than 10 W/cm2. Transient cavitation involves larger variation in the bubble sizes (maximum size reached by the cavity is few hundred times the initial size) over a time scale of few acoustic cycles. The life time of transient bubble is too small for any mass to flow by diffusion of the gas into or out of the bubble however evaporation and condensation of liquid within the cavity can take place freely. Hence, as there is no gas to act as cushion, the collapse is violent. Bubble dynamics analysis can be easily used to understand whether transient cavitation can occur for a particular set of operating conditions. A typical bubble dynamics profile for the case of transient cavitation has been given in Fig. 2.2. By assuming adiabatic collapse of bubble, the maximum temperature and pressure reached after the collapse can be estimated as follows [2]. 

Transient cavitation bubbles are voids, or vapour filled bubbles, believed to be produced using sound intensities in excess of 10 W cm. They exist for one, or at most a few acoustic cycles, expanding to a radius of at least twice their initial size, (Figs. 2.16 and 2.20), before collapsing violently on compression often disintegrating into smaller bubbles. (These smaller bubbles may act as nuclei for further bubbles, or if of sufficiently small radius (R) they can simply dissolve into the bulk of the solution under the action of the very large forces due to surface tension, 2a/R. During the lifetime of the transient bubble it is assumed that there is no time for any mass flow, by diffusion of gas, into or out of the bubble, whereas evaporation and condensation of liquid is assumed to take place freely. If there is no gas to cushion the implosion... 

Eventually the attendant is reaching the balls quickly enough to return them to the bin just as quickly as the kids can throw them out. At this point, the process continues at a frantic pace but with no net change in the numbers of balls in the bin and on the floor. This is like the dynamic equilibrium reached between the rates of evaporation and condensation of liquid and vapor in a closed container. When the rates are equal, there is no net change in the amount of liquid or vapor in the system. 

Distillation is the process by which two or more liquids, or a solution of liquids and solids, are separated as a result of differences in vapor pressure and boiling points through the evaporation and condensation of the mixture components above the boiling liquid mixture. 

It is observed in table 7.2 that the most important terms in the reduction to standard states are the decomposition of aqueous nitric acid (AU (,), the compression of the initial gaseous phase (02+H20) from a negligibly small pressure to the initial pressure (A 6/7), the decompression of the final gaseous phase (02 + N2 + C02 + H20) from the final pressure to a negligibly small pressure (Af/19), and the evaporation of C02 from the final aqueous phase (AU o). The terms relative to the vaporization and condensation of liquid water (A f/3 — AIJ24) almost cancel out. 

Gases and liquids permeate fluoropol5miers to different extents depending on variables such as temperature, pressure, and the composition of the processing fluid. An increase in temperature accelerates the rate of permeation into the polymers. Thermal cycling can cause a part to stress-crack or form blisters because of successive evaporation and condensation of permeated chemicals. Steam is a well-known permeant of polytetrafluoroethylene and can create blisters upon cycling. 

The model suggested by Bertrand et al. [11-13] assumed the existence of a spatial gradient of temperature in the reaction zone. In this model the abnormal rise of the dehydration rate with was attributed to the increase of heat transfer from the furnace to the self-cooled reactant. Model calculations and experiments on the evaporation and condensation of ethanol and water vapours provided a convincing proof of this mechanism. In the experiments [13], the temperature of the evaporating liquids turned out to be much lower than that of the heater. For instance, for ethanol the difference from the thermostat temperature (300 K) was as much as 45 K or 15%. However, this model remained unclaimed during the following 20 years of studies on the T-S effect. Such a considerable difference in temperatures between the crystalline hydrate and the furnace seemed improbable to the majority of researchers. 

In a closed container pardy filled with water the air over the fiquid soon becomes mixed with water vapor. As the air starts to become saturated with water vapor, some of the water vapor starts to condense. Eventually, the evaporation and condensation of the water estabUsh a dynamic equilibrium—a. state of balance between exactly opposite changes occurring at the same rate. To indicate this equilibrium, a double arrow is placed between the symbols for water in the liquid and vapor states (Figure 8.7) ... 

In a confined area above a liquid, equilibrium is established between the evaporation and condensation of molecules from the liquid. For a given temperature, this results in a steady-state level of the vapor, which can be described as a pressure. Such a pressure is called the vapor pressure of the liquid (Figure 2.8). 

Because of the possibility of substantial temperature effects, allowance must be made for evaporation and condensation of the liquid solvent. For this problem, therefore, the components are defined as follows ... 

The separation of close boiling components in a mixture has been accomplished commercially by cascading several evaporations and condensations of such mixtures in a device known as a distillation or rectification column. There are two types of columns, namely, packed columns and plate columns. The former are vertical cylinders filled with a variety of packings that provide a large surface area per unit volume to promote maximum contact between the downward liquid flow and the upward vapor flow. Since this type of column can encounter poorer vapor-liquid contact than the plate column, it is only seldomly used in cryogenic separation systems. 

L2, etc. As the vapor moves up the column, its composition moves to the right along the dew-point curve (points V3, V4, V5, etc.) Note that both components may be separated into very high purity products by using a sufficient number of plates, which serve as vapor-liquid contact stages and thus permit successive evaporation and condensation of part of the mixture to occur in one self-contained piece of equipment. 

The basic assumption is that the Langmuir equation applies to each layer, with the added postulate that for the first layer the heat of adsorption Q may have some special value, whereas for all succeeding layers, it is equal to Qu, the heat of condensation of the liquid adsorbate. A furfter assumption is that evaporation and condensation can occur only from or on exposed surfaces. As illustrated in Fig. XVII-9, the picture is one of portions of uncovered surface 5o, of surface covered by a single layer 5, by a double-layer 52. and so on.f The condition for equilibrium is taken to be that the amount of each type of surface reaches a steady-state value with respect to the next-deeper one. Thus for 5o... 

In addition to volume changes the effect of temperature is also important. Thus the specific latent heat of vaporization of a chemical is the quantity of heat, expressed as kJ/kg, required to change unit mass of liquid to vapour with no associated change in temperature. This heat is absorbed on vaporization so tliat residual liquid or tlie sunoundings cool. Alternatively an equivalent amount of heat must be removed to bring about condensation. Thus the temperature above a liquefied gas is reduced as tlie liquid evaporates and tlie bulk liquid cools. There may be consequences for heat transfer media and the strength of construction materials at low temperatures. 

Yet, it is reasonable to suppose that water molecules from the liquid are still evaporating, even at equilibrium. Molecules in the liquid have no way of knowing that the partial pressure of the vapor is equal to the vapor pressure. In the gas phase, the randomly moving molecules continue to strike the surface of the liquid and condense. Equilibrium corresponds to a perfect balance between this continuing evaporation and condensation. Then no net changes can be detected. ... 

Figure 12-11 is a molecular view of how a solute changes this liquid-vapor equilibrium of the solvent. The presence of a solute means that there are fewer solvent molecules at the surface of the solution. As a result, the rate of solvent evaporation from a solution is slower than the rate of evaporation of pure solvent. At equilibrium, the rate of condensation must be correspondingly slower than the rate of condensation for the pure solvent at equilibrium with its vapor. In other words, the vapor pressure drops when a solute is added to a liquid. A solute decreases the concentration of solvent molecules in the gas phase by reducing the rates of both evaporation and condensation. 

Fractional distillation is the same as distillation, except that a fractionating column (a tube containing a high-surface-area material) is positioned above the boiling liquid mixture such that continuous evaporation and condensation occur with time, resulting in a cleaner separation of all components. 

The next more complicated treatment of liquid water is to have a way in which to model also its transport without going to a two-phase model. The models of this sort assume that the liquid water exists as droplets that are carried along in the gas stream. - - Thus, while evaporation and condensation occur, a separate liquid phase does not have to be modeled. Instead, the liquid is assumed to be a component of the gas, and usually one that has a negligible effect on the gas-phase flow and velocity. There is a change in the gas-phase volume fraction due to the water, however. This type of model allows for the existence and location of liquid water to be noted, and to a limited extent the change in the water pressure or concentration. 

I Distillation allows the separation of a mixture of liquids by evaporation and condensation. It can also be used to Identify compounds. 

In contrast to these we have the equilibrium processes of sublimation, absorption, dissolution, precipitation, evaporation, and condensation, throngh which the physical states of solid, Uqnid, and gas are connected. For example, the common crystallization of salts from sea water involves all three phases. Distillation, which is essential for prodncing organic solvents, is a two-step evaporation (liquid => gas) condensation (gas => Uqnid) process. 

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