Evaporative adiabatic

Based on differences in melting points and Hquid-phase solubilities four modes of operation possible drown-out, isothermal evaporation, adiabatic evaporation, and cooling (choice depends on stream characteristics). 

Figure 4. Schematic of triangular grooves (evaporator, adiabatic zone and condenser) in MHP, [25].

Wet-bulb temperature is the equilibrium temperature reached by a small amount of liquid evaporating adiabatically into a large volume of gas. Equilibrium exists when the rate of heat transfer from the gas to the cooler liquid equals that consumed by evaporation. It is affected by heat and mass transfer coefficients as well as humidity, therefore is dependent on maintaining turbulent gas flow around the bulb. Humidity can be determbied from wet-bulb, and dry-bulb, T, temperatures by following the adiabatic-saturation curves on a psychrometric chart, or by... 

Spray drying involves pumping a slurry into hot air. The liquid trapped between particles in the slurry evaporates adiabatically, normally in a lot of air. Air flow can be concurrent, countercurrent, or a mixture of the two. The process is used to make dried milk, dried eggs, and dry grain. It is the basis of manufacturing instant coffee and laundry detergent. 

Genera.1 Ca.se, The simple adiabatic model just discussed often represents an oversimplification, since the real situation implies a multitude of heat effects (/) The heat of solution tends to increase the temperature and thus to reduce the solubihty. 2) In the case of a volatile solvent, partial solvent evaporation absorbs some of the heat. (This effect is particularly important when using water, the cheapest solvent.) (J) Heat is transferred from the hquid to the gas phase and vice versa. (4) Heat is transferred from both phase streams to the shell of the column and from the shell to the outside or to cooling cods. 

Order of preference adiabatic evaporation > isothermal evaporation > cooling >drown-out. 

Solution crystalli tion (adiabatic evaporation (vacuum cooling))... 

The specific enthalpies ia equation 9 can be determined as described earUer, provided the temperatures of the product streams are known. Evaporative cooling crystallizers operate at reduced pressure and may be considered adiabatic (Q = 0). As with of many problems involving equiUbrium relationships and mass and energy balances, trial-and-error computations are often iavolved ia solving equations 7 through 9. 

Evaporative efficiency in a direct-heat dryer compares vaporization obtained to that which would be obtained if the drying gas were saturated adiabatically. 

Note that during the cooling step the maximum amount of evaporation which is permitted by the material balance is 50 lb for the step shown. In an evaporative-cooling step, however, the actual evaporation which results from adiabatic cooling is more than this. Therefore, water must be added back to prevent the NaCl concentration from rising too high otherwise, coprecipitation of NaCl will occur. 

If the crystallizer is not of the evaporative type but rehes only on adiabatic evaporative cooling to achieve the yield, the heating element is omitted. The feed is admitted into the circulating hue after withdrawal of the slurry, at a point sufficiently below the free-hquid surface to prevent flashing during the mixing process. 

Many of the warmer climates have a dry atmosphere (see Figure 23.8). In such areas, considerable dry bulb temperature reduction can be gained by the adiabatic saturation cycle (Section 24.4). The apparatus draws air over a wetted pad and discharges it into the conditioned space. It is termed an evaporative or desert cooler (Figure 25.3). 

A mixture of equal weights of water and steam at 100° is expanded adiabatically in a cylinder. Prove that some water will evaporate. 

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. 

When humidification is carried out in a packed column, the water which is not evaporated can be recirculated so as to reduce the requirements of fresh water. As a result of continued recirculation, the temperature of the water will approach the adiabatic saturation temperature of the air, and the air leaving the column will be cooled — in some cases to within 1 deg K of the temperature of the water. If the temperature of the air is to be maintained constant, or raised, the water must be heated. 

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. 

Under special conditions sulfur cations with up to 56 atoms have been observed [209]. Evaporation of liquid sulfur and cooling the vapor in an atmosphere of a cold buffer gas (He) at low pressures followed by adiabatic expansion into the vacuum of a mass spectrometer and El ionization produced mass spectra of clusters of sulfur molecules with m/e ratios up to ca. 1800. The intensity pattern shows that the species (Ss)h are most abundant n = 1-7) followed by (Sy)(S8)n-i clusters and (S6)(Ss)h-i clusters. The latter have the same mass as (Sy)2(S8) -2 clusters see Fig. 34. Thus, the composition of the clusters reflects the composition of hquid sulfur near the melting point which contains Sg, Sy and Se molecules as the majority species [34, 210]. 

Fig. 34 Mass spectrum of sulfur clusters obtained by evaporation of liquid sulfur followed by cooling and adiabatic expansion of the vapor [209]. The figures at the peaks give the number of Sg molecules which represent this particular mass...

Adiabatic heat storage or accumulation tests are performed to obtain data on temperature-and pressure-time behaviour of a substance at quasi-adiabatic conditions. Where heat dissipation by evaporation is anticipated, the measurements have to be performed in a closed system. If this is not the case the experiment may be carried out in an open system. 

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