Vaporization, heat dependent

For liquid vaporization conditions at or above the critical point, the rate of vapor discharge depends on the rate at which the fluid will expand. For such situations a latent heat of 116 kJ/kg can be used. 

The vaporization heat of water, which depends on the humidity, is accurately determined by... 

Bo = q/Gh] Q, where t is the period between successive events, U is the mean velocity of single-phase flow in the micro-channel, Jh is the hydraulic diameter of the channel, q is heat flux, m is mass flux, /zlg is the latent heat of vaporization). The dependence t on Bo can be approximated, with a standard deviation of 16%, by... 

The Carnot cycle is a reversible cycle. Reversing the cycle will also reverse the directions of heat and work interactions. The reversed Carnot heat engine cycles are Carnot refrigeration and heat pump cycles. Therefore, a reversed Carnot vapor heat engine is either a Carnot vapor refrigerator or a Carnot vapor heat pump, depending on the function of the cycle. 

DISTILLATION A heat-dependent process used to produce alcoholic beverages, such as whiskey, rum, and vodka. In this process, a fermented mash (of grains, vegetables, or fruits) is heated in a boiler, causing the alcohol to evaporate. The alcohol vapors are then collected and cooled in a condenser to produce the beverage. 

Liquids are constantly evaporating at their surface. That is, the molecules at the surface of the liquid can achieve enough kinetic energy to overcome the forces between them and they can move into the gas phase. This process is called vaporization or evaporation. As the molecules of the liquid enter the gas phase, they leave the liquid phase with a certain amount of force. This amount of force is called the vapor pressure. Vapor pressure depends upon the temperature of the liquid. Think about a pot of water that is being heated in preparation for dinner. The water starts out cold and you do not see any steam. As the temperature of the water increases you begin to see more steam. As the temperature of the water molecules increases, the molecules have more kinetic energy, which allows them to leave the liquid phase with more force and pressure. You can then conclude that as the temperature of a liquid increases, the vapor pressure increases as well. This is a direct relationship. 

The Heat of Vaporization. The molal heat of vaporization represents the energy that must be supplied to vaporize one mole of the liquid. For example, 9714 calories must be supplied to vaporize one mole of water at 100° C. The heat of vaporization does depend upon the temperature at which the vaporization is carried out. However, the variation is in general hot great, so that the assumption made in the previous section regarding the constancy of is justifiable over a small temperature range. 

While all vapor techniques depend on a condensation step, the direct vaporization techniques begin with the desired composition and evaporate or sublimate the material. This is straightforward and allows a diversity of compositions to be used, but excessively high temperatures are demanded to vaporize refractory ceramics. Various heating methods used, including dc arcs, dc plasmas, rf plasmas, and electron beam heating. These techniques are not popular for large-scale production or routine laboratory powder preparation. 

Boiling and vaporizing heat transfer coefficients depend very strongly on the nature of the surface and the structure of the two-phase... 

The overall heat-transfer coefficient, related to the instantaneous total area of the rising two-phase drop, increases sharply with evaporation up to 3-10 wt-% vapor content, depending on the system and conditions, and then decreases quite moderately until evaporation is complete. Thus it indicates the decreasing effect of the internal resistance to heat transfer in the first stages of the evaporation process, and the moderate decrease in the relative transfer area in the subsequent stages of the process (SI2). The instantaneous overall heat-transfer coefficients are 200-400 Btu/hr/ft /°F for D =3.5 mm, and 500-700 Btu/hr/ft /°F for Z> = 2.0 mm. 

The typical GC syringe injection volume of 0.5-2 pL of liquid solvent typically expands to 0.5-1.5mL vapor volume, depending on injector temperature. This is a volume increase on the order of lOOOx. A capillary GC column is operated at a much lower flow rate than a packed column (typically on the order of 1 mL/min instead of 20-40 mL/min). The handling of the volume of vaporized solvent and analyte(s) in capillary GC required a more complex control of gas flows in and out of the injector volume. At the typical capillary column flow rates, it may require a minute or more for all this vapor volume to be transferred to the column. In order that the analytes (if not the solvent) should not spread out after entering the column, the column oven must be maintained at a temperature much lower than that used to vaporize the sample in the injector space. This requires that a separate injector port volume be independently heated to a different and higher temperature than the initial oven temperature. 

As state functions, Cv and Cp depend on pressure and temperature. This dependence is illustrated for Cp in Figure 3-8. which shows the constant-pressure heat capacity of water as a function of pressure at selected temperatures. In general, the heat capacity of the liquid is higher than that of the vapor heat capacity is a strong function of pressure in the vapor phase but almost independent of pressure in the liquid. In both phases, Cp is fairly sensitive on temperature. Following an isotherm to zero pressure we... 

The activation energy determined for the evaporation process is of the same magnitude as the enthalpy of vaporization, and depends closely upon the pressure. At atmospheric pressure and an equal heating rate, the temperature of the evaporation maximum of the DSC curve is found at the same temperature as the corresponding maximum in the DTG curve. The same behavior has been found for the temperatures of the DSC crack maximum and the corresponding maximum in the DTG curve (both at equal heating rates). 

A constant, = breakdown field, t = pulse length, m = mass of liquid, Cp ss specific heat, = boiling point, ambient temperature, and 1 = heat of vaporization. The dependence on pressure and molecular structure (which change the boiling point of liquids) fitted well with this theory. These results were, however, limited to a series of alkanes, and there is no experimental proof that bubble generation is the effective mechanism. 

The pressure drop together with the vapor flow settles virtually instantaneously. The pressure drop consists of the diy and wet pressure drop. Tlie vapor flow depends on the energy balance. Roughly, the leaving vapor flow equals the arriving vapor flow minus condensation due to heat losses. 

The heat transfer characteristics of a solid material are measured by a property called the thermal conductivity (k) measured in Btu/h ft °F. It is a measure of a substance s ability to transfer heat through a solid by conduction. The thermal conductivity of most liquids and solids varies with temperature. For vapors, it depends upon pressure. Thermal conductivity varies with temperature but not always in the same direction. 

The temperature dependence of many properties of methanol has been described in figures, tables, and equations. Plots of vapor pressure, liquid density, liquid heat capacity, vapor heat capacity, heat of vaporization, surface tension, liquid thermal conductivity, vapor thermal conductivity, liquid viscosity, and vapor viscosity against temperature have been given by Yaws [13] and by Flick [14]. Tables of vapor pressure [3,1517], liquid density [3,15,17], liquid volume [16], vapor density [15,17], vapor volume [16], liquid viscosity [15,18], vapor viscosity [15], surface tension [15,19], liquid heat capacity [15,17,20], vapor heat capacity [3,15,17], solid heat capacity [11], liquid thermal conductivity [15,17], vapor thermal conductivity [15], second viral coefficient [16], dielectric constant [21], refractive index [3], and heat of vaporization [16] have also been published. Thermodynamic properties of methanol in the condensed phases have been tabulated by Wilhoit et al. [11], and those in the gas phase have been given by Chao et aL [9]. 

When multiple analyte species are present in the sample, each has a characteristic atomization rate dependent on the vapor pressure of the material. This results in different species being vaporized at different times. Figure 5.22 shows signal responses for Cd, Ni, and Cr in a single sample deposited in an ETV Cadmium has the lowest vaporization temperature and, as predicted, is the first of the three elements to be vaporized from the ETV. This process is followed by Ni and Cr, in the order of their vaporization temperatures. Depending on the heating rates and the physical and chemical characteristics of each analyte, various elements may vaporize from the heated surface at different times therefore, optimization for each type of sample is required. Use of matrix modifiers, as in GFAAS, can stabilize this process. 

The transition of a liquid phase to its vapor phase involves the separation of molecules in the liquid and the removal of molecules from the surface of the liquid into the vapor phase. The energy absorbed when a definite quantity of a liquid is vaporized (the latent heat of vaporization) therefore depends on the intermolec-ular attractive forces which have to be overcome in order to separate molecules. According to Trouton s rule, the boiling points of nonassociated liquids, on the absolute-temperature scale, are approximately proportional to their latent heats of vaporization. Hence, the boiling point of a liquid depends on the relative strength of cohesive intermolecular forces. 

Accurate enthalpies of solid-solid transitions and solid-liquid transitions (fiision) are usually detennined in an adiabatic heat capacity calorimeter. Measurements of lower precision can be made with a differential scaiming calorimeter (see later). Enthalpies of vaporization are usually detennined by the measurement of the amount of energy required to vaporize a known mass of sample. The various measurement methods have been critically reviewed by Majer and Svoboda [9]. The actual teclmique used depends on the vapour pressure of the material. Methods based on... 

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