Evaporation, rate

Evaporation of solvents from a wet film takes place in essentially two stages initially, the solvent loss is primarily dependent on the volatility (related to vapor pressure) of the solvent, and at the later stage, as film formation proceeds, solvent loss by evaporation become increasingly difficult and essentially becomes diffusion controlled, a slow process. This latter stage may be dominant when as much as 20 % of the solvent is retained in the film. 

In general, among the important factors affecting evaporation rate of a solvent from a wet film are vapor pressure of the solvent, temperature, the air flow rate at the surface, and the wet film thickness. 

Measurement of evaporation rates is done very simply by a gravimetric method. Shell developed an automatic evaporometer described in ASTM D 3539. Two methods are commonly used evaporation from a thin film of the liquid, or evaporation from filter paper, using equal volumes of liquids in each case. The filter paper method is preferred. The evaporation time of a given amount of solvent is determined experimentally under identical conditions and compared 

With that of butyl acetate or diethyl ether. The relative evaporation rate is defined as  

E (butyl acetate) = t (butyl acetate) /1 (test solvent) 

The evaporation rate can be derived either (1) by a plot of time versus weight using a solvent having a known evaporation rate for comparison or (2) from the distillation profile (ASTM D86 IP 123). Although the results obtained on the naphtha provide a useful guide, it is, wherever possible, better to carry out a performance test on the final product when assessing enviromnental effects. 

The evaporation rate is an important property of naphtha, and although there is a significant relation between distillation range and evaporation rate, the relationship is not straightforward. 

A simple procedure for determining the evaporation rate involves use of at least a pair of fared shallow containers, each containing a weighed amount of naphtha. The cover-free containers are placed in a temperature-and humidity-controlled draft-free area. The containers are reweighed at intervals until the samples have completely evaporated or have left a 

The evaporation rate can be derived either (1) by a plot of time versus weight using a solvent having a known evaporation rate for comparison or 

Water has a slow evaporation rate, much slower than solvents with a similar boiling range. Solvents such as toluene and xylene evaporate 8-10 times faster. 

An increase in relative humidity does not affect the evaporation rate of xylene (xylol) but with water the evaporation rate decreases with increasing relative humidity. The vapour pressure of water increases with increasing temperature, hence the atmosphere above the coating system holds more moisture (solvent) as the temperature is increased and thus greater air flow rates are required for efficient flash off and stoving times. 

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For vapor saturated with respect to liquid water at room temperature, Z is about 0.02 mol/cm sec or about 1.2 X 10 molecules/cm sec. At equilibrium, then, the evaporation rate must equal the condensation rate, which differs from... 

Some fairly typical results, obtained by LaMer and co-workers [275] are shown in Fig. IV-24. At the higher film pressures, the reduction in evaporation rate may be 60-90%—a very substantial effect. Similar results have been reported for the various fatty acids and their esters [276,277]. Films of biological materials may offer little resistance, as is the case for cholesterol [278] and dimyristoylphosphatidylcholine (except if present as a bilayer) [279]. 

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]. 

Droplet trajectories for limiting cases can be calculated by combining the equations of motion with the droplet evaporation rate equation to assess the likelihood that drops exit or hit the wall before evaporating. It is best to consider upper bound droplet sizes in addition to the mean size in these calculations. If desired, an instantaneous value for the evaporation rate constant may also be used based on an instantaneous Reynolds number calculated not from the terminal velocity but at a resultant velocity. In this case, equation 37 is substituted for equation 32 ... 

Acryhc and methacryhc nonaqueous dispersions (NADs) are primarily utilized by the coatings industry to avoid certain difficulties associated with aqueous dispersion (emulsion) polymers. Water as a suspension medium has numerous practical advantages, but also some inherent difficulties a high heat of evaporation, a low boiling point, and an evaporation rate that depends on the prevailing humidity. Nonaqueous dispersions alleviate these problems, but introduce others such as flammabihty, increased cost, odor, and toxicity. 

Solvents. Solvents in house paints serve several essential purposes. They keep the binder dispersed or dissolved and the pigments dispersed in an easy-to-use state. Solvents allow the paint to be appHed in the correct thickness and uniformity, and evaporate from the paint film after the paint is apphed. Solvent choice is limited mainly to a solvent that is compatible with the binder system and that has the desked evaporation rate and toxicity profile. The volatility or evaporation rate of a solvent determines to a large extent the open-time and dry-time properties of a paint (6). 

Mineral spirits, a type of petroleum distillate popular for use in solvent-based house paints, consist mainly of aUphatic hydrocarbons with a trace of aromatics. This type of solvent finds use in oil- and alkyd-based house paints because of its good solvency with typical house paint binders and its relatively slow evaporation rate which imparts good bmshabiUty, open-time, and leveling. Other properties include lower odor, relatively lower cost, as well as safety and health hazard characteristics comparable to most other organic solvents. 

Alloys. Alloys consist of two or mote elements of different vapor pressures and hence different evaporation rates. As a result, the vapor phase and therefore the deposit constantiy vary in compositions. This problem can be solved by multiple sources or a single rod- or wire-fed electron beam source fed with the alloy. These solutions apply equally to evaporation or ion-plating processes. 

The maximum congment sublimation temperature, corresponds to the temperature above which the anion evaporation rate exceeds the cation evaporation rate from a (001) crystal surface in a vacuum. 

The evaporation rate from the gel is dictated by the difference between the vapor pressure at the evaporating surface, P, and the vapor pressure of the ambient atmosphere, P. Evaporation continues as long as at a rate, E), of... 

Solvent Selection. A thorough knowledge of the requkements of each solvent appHcation is necessary to formulate a solvent system successfully and meet all needs at the lowest possible cost. The most important properties are solvency, evaporation rate, flash poiat, and solvent balance. In nearly every appHcation, these properties are important even though the specific requkements differ greatly from one appHcation to another. Each potential solvent has a particular set of properties, and the solvent chosen and the amount of each depend on the specific appHcation requkements. 

Flash Point. As a liquid is heated, its vapor pressure and, consequendy, its evaporation rate increase. Although a hquid does not really bum, its vapor mixed with atmospheric oxygen does. The minimum temperature at which there is sufficient vapor generated to allow ignition of the air—vapor mixture near the surface of the hquid is called the dash point. Although evaporation occurs below the dash point, there is insufficient vapor generated to form an igrhtable mixture below that point. 

Therefore, 12.37 kg saline water are needed in this case to produce 1 kg distillate. This high dow rate incuts corresponding pumping equipment and energy expenses, sluggish system dynamics, and, because the stream level depth is limited to about 0.3—0.5 m for best evaporation rates, also requites large evaporator vessels with their associated expense. 

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