Liquids evaporation

The chemical potential of a curved surface is extremely critical in ceramic processing. It detemiines reactivity, tlie solubility of a solid in a liquid, tire rate of liquid evaporation from solid surfaces, and material transport during sintering. 

If the water insoluble mixture is a liquid, evaporate a small sample (say, 4 ml.) in an evaporating dish on a water bath in order to determine the amount of volatile components, if any. If the solvent distils at the temperature of the boihng water bath, it is advisable to distil ofiF this solvent on a water bath and to replace it by ether. 

The liquefied gas must be maintained at or below its boiling point. Refrigeration can be used, but the usual practice is to cool by evaporation. The quantity of liquid evaporated is minimized by insulation. The vapor may be vented to the atmosphere (wasteful), it may be compressed and reliquefied, or it may be used. 

In most diying operations, water is the liquid evaporated and air is the normally employed purge gas. For diying purposes, a psychro-metric chart found very useful is that reproduced in Fig. 12-36. 

Tti e wet-bulb temperature is established by a dynamic equilibrium between heat and mass transfer when liquid evaporates from a small mass, such as the wet bulb of a thermometer, into a veiy large mass of gas such that the latter undergoes no temperature or humidity change. It is expressed by the relationship... 

Unconfined Vapor Cloud Explosions (UVCEs) and Boiling Liquid Evaporating Vapor Explosions (BLEXT s)... 

Material stored at a reduced temperature has little or no superheat and there will be little flash in case of a leak. Vaporization will be mainly determined by liquid evaporation from the surface of the spilled liqrtid, which depends on weather conditions. 

When a liquid is dispersed into droplets the surface area is increased, which enhances the rates of heat and mass transfer. For a particular liquid dispersed at constant concentration in air the MIE varies with approximately the cube of surface average droplet diameter, hence the MIE decreases by a factor of about 8 when the surface average diameter D is halved (A-5-1.4.4). Ease of ignition is greatly enhanced for finely divided mists with D less than about 20 /rm, whose MIE approaches that of the vapor. Below 10 /rm a high flash point liquid mist (tetrahydronaphthalene) was found to behave like vapor while above about 40/rm the droplets tended to burn individually [ 142]. Since liquid mists must partially evaporate and mix with air before they ignite, the ease with which a liquid evaporates also affects MIE (Eigure 5-1.4.4). 

The model assumes that liquid evaporation is always the rate controlling step. At some point the model must fail, since as droplet size approaches zero the predicted MIE approaches zero rather than the MIE of the vapor in air. In practice, droplets having diameters less than 10-40 /rm completely evaporate ahead of the flame and burn as vapor (5-1.3). The model also predicts that the MIE continuously decreases as equivalence ratio is increased, although as discussed above, combustion around droplets is not restrained by the overall stoichiometry and naturally predominates at the stoichiometric concentration. It is recommended that the model be applied only to droplet diameters above about 20/rm and equivalence ratios less than about one. 

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. 

In many industrial processes, the many components contained in the liquid evaporate simultaneously. Evaporation of individual components is easy to determine. For multicomponent liquid systems, the individual evaporation rates are summed to obtain the total evaporation rate. 

Heat gains and losses for heating or cooling raw materials and parts brought into or taken out of the building, melted metal solidification, vapor condensation, or liquid evaporation... 

If the pressure on a liquefied gas is reduced, some of the liquid evaporates, and the rest gets colder. All refrigeration plants, domestic and... 

The reaction mixture is removed and about half of the liquid evaporated, an oil separating during the process. The mixture is acidified with concentrated hydrochloric acid and extracted with two 100 cc portions of ether. The extracts, which contain the 5-phenyl-5-(2-thienyOhydantoin, are combined and the combined ether extracts are shaken with two 25 cc portions of 5% potassium hydroxide solution. The alkaline solution, which dissolves the 5-phenvl-5-(2-thienvl)hydantoin to form the potassium salt thereof, is acidifed with hydrochloric acid and heated to expel ether. 

Tampella recovery system, falling film black liquid evaporation, lime kiln modifications... 

In mechanical refrigeration, a multicomponent refrigerant consisting of nitrogen, methane, ethane, and propane is used through a cascade cycle. When these liquids evaporate, the heat required is obtained from... 

Figure 5.5 Change of gas pressure with time as a liquid evaporates into a closed container. The pressure p is the equilibrium vapor pressure.

The liquid evaporating into the gas is transferred by diffusion from the interface to the gas stream as a result of a concentration difference (c0 — < ) where cunit volume) and c is the concentration in (he gas stream. The rate of evaporation is then given by ... 

In the system just considered, neither the humidity nor the temperature of the gas is appreciably changed. If the gas is passed over the liquid at such a rate that the time of contact is sufficient for equilibrium to be established, the gas will become saturated and both phases will be brought to the same temperature. In a thermally insulated system, the total sensible heat falls by an amount equal to the latent heat of the liquid evaporated. As a result of continued passage of the gas, the temperature of the liquid gradually approaches an equilibrium value which is known as the adiabatic saturation temperature. 

Peles el al. (2000) elaborated on a quasi-one-dimensional model of two-phase laminar flow in a heated capillary slot due to liquid evaporation from the meniscus. Subsequently this model was used for analysis of steady and unsteady flow in heated micro-channels (Peles et al. 2001 Yarin et al. 2002), as well as the study of the onset of flow instability in heated capillary flow (Hetsroni et al. 2004). 

The idealized picture of the flow in a heated micro-channel is shown in Fig. 8.1a. Such flow possesses a number of specific properties due to its unique structure, which forms because of liquid evaporation and the interaction of pure vapor and liquid flows separated by the interface surface. The latter has an infinitely thin surface with a jump in pressure and velocity, while the temperature is equal. One can... 

Evaporative two-phase flow in a heated micro-channel resembles a two-phase slug flow with distinct domains of liquid and vapor. These domains are divided by the infinitely thin evaporating front, which propagates relatively to the fluid with a velocity u f equal (numerically) to the linear rate of liquid evaporation. In the frame of reference associated with micro-channel walls, the velocity of the evaporation front is... 

The temperature distribution has a characteristic maximum within the liquid domain, which is located in the vicinity of the evaporation front. Such a maximum results from two opposite factors (1) heat transfer from the hot wall to the liquid, and (2) heat removal due to the liquid evaporation at the evaporation front. The pressure drops monotonically in both domains and there is a pressure jump at the evaporation front due to the surface tension and phase change effect on the liquid-vapor interface. 

The experimental investigation of the flow in a heated capillary shows that, depending on the value of the Peclet number, various types of the process occurred. At small Pcl, the dominant role is the bubble formation at the channel wall, whereas for Pcl > 1, liquid evaporation leads to formation of an evaporation front. 

Choose the characteristic velocity m so, that total heat flux on the wall is fully expended for liquid evaporation (the heating without any losses of heat r = 1). We conclude that... 

Equation (10.50) postulates equality of the velocity due to liquid evaporation u l and the velocity due to the capillary and pressure forces Ml. 

The change of velocify due to liquid evaporation l and influence of the capillary forces L versus Xf for > 1 is illustrated in Fig. 10.6. In the case 2> 1 the curves L(Xf) and L(xf) have only one point of intersection, which determines the stationary values of ml = L.st and Xf = Xf,st. It is not difficult to show that this point is stable. Indeed a displacement of the meniscus from its initial position Xf,st to the position x[ ) leads to the situation, when the velocity due to the liquid evaporation Hl exceeds the velocity due to the capillary force u[. This leads to the return of the meniscus to its initial position. If the meniscus displaces to the left, > u, this also leads to the return of the system to its initial state. 

The wall heat flux is the cause for the liquid evaporation, and perturbation of equilibrium between the gravity and capillary forces. It leads to the offset of both phases (heated liquid and its vapor) and interface displacement towards the inlet. In this case the stationary state of the system corresponds to an equilibrium between gravity, viscous (liquid and vapor) and capillary forces. Under these conditions the stationary height of the liquid level is less than that in an adiabatic case... 

Unlike at adiabatic conditions, the height of the liquid level in a heated capillary tube depends not only on cr, r, pl and 6, but also on the viscosities and thermal conductivities of the two phases, the wall heat flux and the heat loss at the inlet. The latter affects the rate of liquid evaporation and hydraulic resistance of the capillary tube. The process becomes much more complicated when the flow undergoes small perturbations triggering unsteady flow of both phases. The rising velocity, pressure and temperature fluctuations are the cause for oscillations of the position of the meniscus, its shape and, accordingly, the fluctuations of the capillary pressure. Under constant wall temperature, the velocity and temperature fluctuations promote oscillations of the wall heat flux. 

For the study of flow stability in a heated capillary tube it is expedient to present the parameters P and q as a function of the Peclet number defined as Pe = (uLd) /ocl. We notice that the Peclet number in capillary flow, which results from liquid evaporation, is an unknown parameter, and is determined by solving the stationary problem (Yarin et al. 2002). Employing the Peclet number as a generalized parameter of the problem allows one to estimate the effect of physical properties of phases, micro-channel geometry, as well as wall heat flux, on the characteristics of the flow, in particular, its stability. 

It is shown that the stability of the flow, with evaporating meniscus, depends (other conditions being equal) on the wall heat flux. The latter determines the rate of liquid evaporation, equilibrium acting forces, meniscus position, as well as the heat losses to the cooling inlet. The stable stationary flow with fixed meniscus position corresponds to low wall heat fluxes (Pe heat fluxes (Pe > 1) an exponential increase of small disturbances takes place. That leads to the transition from stable stationary to unstable flow with oscillating meniscus. 

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