Rate of evaporation

The physical chemist is very interested in kinetics—in the mechanisms of chemical reactions, the rates of adsorption, dissolution or evaporation, and generally, in time as a variable. As may be imagined, there is a wide spectrum of rate phenomena and in the sophistication achieved in dealing wifli them. In some cases changes in area or in amounts of phases are involved, as in rates of evaporation, condensation, dissolution, precipitation, flocculation, and adsorption and desorption. In other cases surface composition is changing as with reaction in monolayers. The field of catalysis is focused largely on the study of surface reaction mechanisms. Thus, throughout this book, the kinetic aspects of interfacial phenomena are discussed in concert with the associated thermodynamic properties. 

An interesting consequence of covering a surface with a film is that the rate of evaporation of the substrate is reduced. Most of these studies have been carried out with films spread on aqueous substrates in such cases the activity of the water is practically unaffected because of the low solubility of the film material, and it is only the rate of evaporation and not the equilibrium vapor pressure that is affected. Barnes [273] has reviewed the general subject. 

Not all molecules striking a surface necessarily condense, and Z in Eq. VII-2 gives an upper limit to the rate of condensation and hence to the rate of evaporation. Alternatively, actual measurement of the evaporation rate gives, through Eq. VII-2, an effective vapor pressure Pe that may be less than the actual vapor pressure P. The ratio Pe/P is called the vaporization coefficient a. As a perhaps extreme example, a is only 8.3 X 10" for (111) surfaces of arsenic [11]. 

The derivation that follows is essentially that given by Langmuir [9] in 1918, in which one writes separately the rates of evaporation and of condensation. The surface is assumed to consist of a certain number of sites S of which S are occupied and Sq = S - S arc free. The rate of evaporation is taken to be proportional to 5, or equal tokiSi, and the rate of condensation proportional to the bare surface So and to the gas pressure, or equal to k PSo. At equilibrium. 

If the fraction of sites occupied is 0, and the fraction of bare sites is 0q (so that 00 + 1 = 0 then the rate of condensation on unit area of surface is OikOo where p is the pressure and k is a constant given by the kinetic theory of gases (k = jL/(MRT) ) a, is the condensation coefficient, i.e. the fraction of incident molecules which actually condense on a surface. The evaporation of an adsorbed molecule from the surface is essentially an activated process in which the energy of activation may be equated to the isosteric heat of adsorption 4,. The rate of evaporation from unit area of surface is therefore equal to... 

When extended to the second layer, the Langmuir mechanism requires that the rate of condensation of molecules from the gas phase on to molecules already adsorbed in the first layer, shall be equal to the rate of evaporation from the second layer, i.e. 

The rate of evaporation of ions from a heated surface is given by Equation 7.3, in which Q, is the energy of adsorption of ions on the filament surface (usually about 2-3 eV) and Cj is the surface density of ions on the surface (a complete monolayer of ions on a filament surface would have a surface density of about 10 ions/cm" ). 

The previous discussion demonstrates that measurement of precise isotope ratios requires a substantial amount of operator experience, particularly with samples that have not been examined previously. A choice of filament metal must be made, the preparation of the sample on the filament surface is important (particularly when activators are used), and the rate of evaporation (and therefore temperature control) may be crucial. Despite these challenges, this method of surface ionization is a useful technique for measuring precise isotope ratios for multiple isotopes. Other chapters in this book discuss practical details and applications. 

Schematic illustrations of the effect of temperature and surface density (time) on the ratio of two isotopes, (a) shows that, generally, there is a fractionation of the two isotopes as time and temperature change the ratio of the two isotopes changes throughout the experiment and makes difficult an assessment of their precise ratio in the original sample, (b) illustrates the effect of gradually changing the temperature of the filament to keep the ratio of ion yields linear, which simplifies the task of estimating the ratio in the original sample. The best method is one in which the rate of evaporation is low enough that the ratio of the isotopes is virtually constant this ratio then relates exactly to the ratio in the original sample.

The rate of evaporation of solvent from droplets can be increased by blowing a drying gas across the stream. Nitrogen is frequently used as the drying gas. 

Cutback asphalts are mixtures ia which hard asphalt has beea diluted with a lighter oil to permit appHcatioa as a Hquid without drastic heatiag. They are classified as rapid, medium, and slow cuting, depending on the volatiHty of the diluent, which governs the rates of evaporation and consequent hardeniag. 

Some concerns directly related to a tomizer operation include inadequate mixing of Hquid and gas, incomplete droplet evaporation, hydrodynamic instabiHty, formation of nonuniform sprays, uneven deposition of Hquid particles on soHd surfaces, and drifting of small droplets. Other possible problems include difficulty in achieving ignition, poor combustion efficiency, and incorrect rates of evaporation, chemical reaction, solidification, or deposition. Atomizers must also provide the desired spray angle and pattern, penetration, concentration, and particle size distribution. In certain appHcations, they must handle high viscosity or non-Newtonian fluids, or provide extremely fine sprays for rapid cooling. 

Zinc Costing of Ca.pa.citors, In the zinc coating of paper strip for capacitors, the paper strip is fed from air through locks into a vacuum environment. There, it is coated by thermally evaporated zinc. The rate of evaporation is so high that contamination of the zinc vapor is excluded. The paper is fed at the maximum rate permitted by its own strength. 

An important characteristic of solvents is rate of evaporation. Rates of solvent loss are controUed by the vapor pressure of the solvent(s) and temperature, partial pressure of the solvent over the surface, and thus the air-flow rate over the surface, and the ratio of surface area to volume. Tables of relative evaporation rates, in which -butyl acetate is the standard, are widely used in selecting solvents. These relative rates are deterrnined experimentally by comparing the times required to evaporate 90% of a weighed amount of solvent from filter paper under standard conditions as compared to the time for -butyl acetate. The rates are dependent on the standard conditions selected (6). Most tables of relative evaporation rates are said to be at 25°C. This, however, means that the air temperature was 25°C, not that the temperature of the evaporating solvent was 25°C. As solvents evaporate, temperature drops and the drop in temperature is greatest for solvents that evaporate most rapidly. 

Performance data on some typical tray and compartment diyers are tabulated in Table 12-10. These indicate that an overall rate of evaporation of 0.0025 to 0.025 kg water/(s m") of tray area may be expected from tray and tray-truck diyers. The thermal efficiency of this type of diyer will vary from 20 to 50 percent, depending on the diying temperature used and the humidity of the exhaust air. In diying to very low moisture contents under temperature restrictions, the thermal efficiency may be in the order of 10 percent. The major operating cost for a tray diyer is the labor involved in loading and unloading the trays. About two labor-hours are required to load and unload a standard two-truck tray diyer. In addition, about one-third to one-fifth of a... 

The simplest system in which useful products are obtained by thermochemical processing involves the evaporahon of an element or elements in vacuum in order to produce thin hlms on a selected substrate. This process is usually limited to the production of thin hlms because of the low rates of evaporation of the elements into a vacuum under conditions which can be controlled. These rates can be calculated by the application of the kinetic theory of ideal gases. 

The rate of evaporation from the surface of an alloy into the surrounding vacuum, will usually be different for each component and at the temperature T is given by... 

The drying a chemical substance is not a simple process. Drying a mass of finely divided solid particles carrying 30 to 40% water, for example the rate of evaporation is constant and high as long as the surfaces exposed are wet. After the surface is dry, the water in the interstices must make its way to the surface, a process of diffusion that is slower than evaporation from a wet surface the rate will then drop. This second part of the process must be modified according to the case with which the material crumbles as it dries, exposing new surfaces. 

Liquefied ammonia is delivered in rail tankcars to Fisons Limited for storage in a 1,900 tonnes spherical tank at -6° C. Several hundred tonnes of liquefied ammonia could be released on land if either of the two storage tanks, at Shell UK Oil and at Fisons Limited failed. The consequences of f lilure of the Shell tank would be minimal, because a high concrete wall to contain the contents and limit the heat transfer and consequently the rate of evaporation of the liquid. Such protection has not been provided. Because of the storage under pressure there are numerous ways the tank could fail from material defect to missile. The spillage of 50 to 100 tonnes, could kill people if noi [imrnp( , evacuation. 

Eye conditions Possible bacteria buildup and health effects. A high rate of evaporation of the eyeball oil film results in dry, itchy eyes with an increase in eye damage in dusty environments,... 

Verdunstungs-gefdss, n. evaporating vessel evaporimeter. -geschwmdigkeit, /. rate of evaporation, -kalte, /. cold due to evaporation. -kiihlung, /. cooling by evaporation, -messer, m. evaporimeter. -verlust, m. loss by evaporation or vaporization, -warme, /. heat of vaporization, verdutzen, v.t. nonplus. 

E = rate of evaporation, gpm (if not accurately known, evaporation can be approximated by multiplying total water rate in gpm times the cooling range (°F) times 0.0008). 

Rate of evaporation (cooling tower), gpm Ot eraU column efficiency, %... 

The rate of evaporation from a cooling tower is approximately 1 per cent of the circulation rate for each 5°C drop in temperature across the tower, or about 7 liters/h per ton of refrigeration. Windage losses will obviously depend on the prevailing wind conditions and the design of the tower with regard to spray elimination but, typically, these are about 0.2 per cent of the circulation rate. 

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