Water, vaporization, enthalpy

When 0 °C dry air is chosen as the zero point of dry air enthalpy, and 0 °C watet as the zero point of water vapor enthalpy, the enthalpies of dty air and water vapor can be calculated from the equations... 

Although we have focused so far primarily on the thermal energy effects resulting from chemical reactions, many physical processes also involve the absorption or release of heat. Examples include phase transformations, such as the melting of ice or the condensation of water vapor. Enthalpy changes occur as well when a solute dissolves in a solvent or when a solntion is diluted. 

Water-vapor enthalpy, averaged over 212 to 260° F Solids temperature during constant-rate drying Product moisture at start of falling-rate drying... 

The integration can be carried out graphically or numerically using a computer. For illustrative purposes the graphical procedure is shown in Figure 5. In this plot of vapor enthalpy or FQ vs Hquid temperature (T or T, the curved line is the equiHbtium curve for the system. For the air—water system, it is the 100% saturation line taken direcdy from the humidity diagram (see Fig. 3). 

Molecular Nature of Steam. The molecular stmcture of steam is not as weU known as that of ice or water. During the water—steam phase change, rotation of molecules and vibration of atoms within the water molecules do not change considerably, but translation movement increases, accounting for the volume increase when water is evaporated at subcritical pressures. There are indications that even in the steam phase some H2O molecules are associated in small clusters of two or more molecules (4). Values for the dimerization enthalpy and entropy of water have been deterrnined from measurements of the pressure dependence of the thermal conductivity of water vapor at 358—386 K (85—112°C) and 13.3—133.3 kPa (100—1000 torr). These measurements yield the estimated upper limits of equiUbrium constants, for cluster formation in steam, where n is the number of molecules in a cluster. 

In Figure 2 the lines, volume, m /kg dry air, indicate humid volume, which includes the volume of 1.0 kg of dry gas plus the volume of vapor it carries. Enthalpy at saturation data are accurate only at the saturation temperature and humidity however, for air—water vapor mixtures, the diagonal wet bulb temperature lines are approximately the same as constant-enthalpy adiabatic cooling lines. The latter are based on the relationship ... 

The diagram in Fig. 11-101 presents enthalpy data for LiBr-water solutions. It is needed for the thermal calculation of the cycle. Enthalpies for water and water vapor can be determined from the table or properties of water. The data in Fig. 11-101 are apphcable to saturated or subcooled solutions and are based on a zero enthalpy of liquid water at 0°C and a zero enthalpy of solid LiBr at 25°C. Since... 

The heat requirements in batch evaporation are the same as those in continuous evaporation except that the temperature (and sometimes pressure) of the vapor changes during the course of the cycle. Since the enthalpy of water vapor changes but little relative to temperature, the difference between continuous and batch heat requirements is almost always negligible. More important usually is the effect of variation of fluid properties, such as viscosity and boiling-point rise, on heat transfer. These can only be estimated by a step-by-step calculation. 

From air-water vapor-mixture tables, the enthalpy hi of the ambient air at 78 F wet-bulb temperature is 41.58 Btu/lb. 

We denote again the mass of dry air in a volume V as w, and the mass of water vapor as mf,. When humid air is treated as an ideal mixtute of two components, dry air and water vapor, the enthalpy of this mixtute is... 

Technical calculations dealing with humid air are reasonable to solve with dry air mass flow rates, because these remain constant in spite of changes in the amount of water vapor in the air. For that reason a definition for enthalpy,... 

In calculations with humid air, when the pressure is not high (usually the atmosphetic pressure of 1 bar), water vapor and dry air can be handled as an ideal gas, as we have already done in Eqs. (4.76) and (4.78). For ideal gases the specific enthalpy is just a function of tempetatute ... 

Thus the total mass flows tn= m, + m,) differ in different cases. Water vapor flow th, is obtained by multiplying the dry air mass flow by the corresponding humidity x (Eq. 4.93). As a basic quantity in humid air mass and energy balance calculations, we use dry air mass flow m and the effect of humidity on the energy balance is noted in the enthalpy h, (Eq. 4.87). 

Temperature rc) Humidity kg HjO/kg dry air) Water vapor partial pressure (kPa) Water v K>r partial density (kg/m ) Water vaporization heat M/kg) Mixture enthalpy (kj/kg dry air) Dry air partial density (lKinematic viscosity (I0< mJ/s) Specific heat (kJ/K kg) Heat conductivity (W/m K) Diffusion factor water air (1 O mJ/s) Temperature rc)... 

The total furnace heat absorption may be estimated by using the calculated furnace exit gas temperature and analysis to determine the enthalpy (excluding the latent heat of water vapor) and thus deducting the heat rejection rate from the net heat input rate. 

Gas fuel contains more hydrogen than coal, thus giving rise to more water vapor in the boiler-stack gases and a relative reduction in thermal efficiency (enthalpy loss), due to the cooling effect of the water. 

NOTE Economizers are, in fact, fitted to some gas-fired, packaged FT boilers. The economizer is designed to recover some of the enthalpy lost as a result of the increased water vapor content in the exit gases from burning natural gas. 

NOTE Hj. = enthalpy of saturated water, Hjg = enthalpy of vaporization, Hg =... 

Sublimation is the direct conversion of a solid into its vapor. Frost disappears on a cold, dry morning as the ice sublimes directly into water vapor. Solid carbon dioxide also sublimes, which is why it is called dry ice. Each winter on Mars, solid carbon dioxide is deposited as polar frost, which sublimes when the feeble summer arrives (Fig. 6.24). The enthalpy of sublimation, AHsub, is the molar enthalpy change when a solid sublimes ... 

In the first reaction, the water is produced as a vapor in the second, it is produced as a liquid. The heat generated is different in each case. We have already seen that the enthalpy of water vapor is 44 kj-mol 1 higher than that of liquid water at 25°C (see Table 6.3). As a result, an additional 88 kj (for 2 mol H20) remains stored in the system if water vapor is formed (Fig. 6.28). If the 2 mol H20(g) subsequently condenses, an additional 88 kj is given off as heat. 

Estimate the enthalpy change of the reaction between gaseous iodoethane and water vapor ... 

C14-0133. The enthalpy of sublimation of Ice at 273.15 K Is not the simple sum of the enthalpies of fusion and vaporization of water, but it can be calculated using Hess law and an appropriate path that Includes fusion and vaporization. Devise such a path, show it on a phase diagram for water, and carry out the calculation, making reasonable assumptions If necessary (C(liquid water) = 75.3 7 mol K , and C(water vapor) = 33.6 K ). 

The equation shows the change in enthalpy when one mole of liquid water vaporizes into water vapor. This is called the molar heat of vaporization. Given this information, which of these is the proper value for the molar heat of condensation ... 

At a given ambient water vapor pressure (usually the level found in the open atmosphere), the temperature of the material is raised so that the equilibrium water vapor pressure over the hydrated material is higher than the ambient water vapour pressure. Generally, heating up to 400 °C is sufficient to remove all the water of crystallization from materials. This removal of water yields a material which may contain some more strongly bound water. To remove this water, the material requires to be heated to a higher temperature (400-600 °C) so that the equilibrium water vapour pressure exceeds the ambient water vapour pressure. For near-complete removal of the last traces of water, temperatures as high as 1000 °C may be required. In addition to the heat required to raise the temperature of the material, heat is also required for the evaporation of water, which is an endothermic process. The enthalpy of evaporation increases as the water content, and hence the equilibrium water vapor pressure, decreases. 

The slope of the line allows for the determination of the enthalpy of vaporization of water, A//Vap, and the y intercept yields the entropy of vaporization, A. S vap As both the enthalpy and the entropy of water increase as the phase change liquid — vapor occurs, the slope and y intercept of the Clausius-Clapeyron equation are negative and positive, respectively. At 373 K these thermodynamic quantities have values of AHvap = 40.657 kJ mol-1 and ASvap = 109.0 J K-1 mol-1. The leavening action due to water vapor or steam arises from the increased amount of water vapor that forms as pastry temperatures initially rise in the oven and then from the increased volume of the water vapor as temperatures continue... 

The standard molar enthalpy of formation, A// , is the amount of heat absorbed when 1 mole of the substance is produced from its elements in their standard states. At 25°C, A// of liquid water is -285.8 kJ/mol and A// of water vapor is -241.8 kJ/mol. This means that more heat is released when liquid water is formed from its elements, then when gaseous water is formed from its elements. So, the formation reaction of liquid water is... 

Determine by calculation the enthalpy of formation in kJ/mole of CH4 given that its heat of combustion is 50.0 kJ/g at 25 °C. The heat of formation for carbon dioxide is -394 kJ/mole and water vapor is -242 kJ/mole. 

Worked Example 3.8 A mole of water vaporizes. What is the change in enthalpy, A HI... 

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