Latent enthalpy effects

Topics to be addressed include temperature, pressure, moles and molecular weights, mass and volume, viscosity, heat capaci, thoinal conductivity, Reynolds number, pH, vapor pressure, ideal gas law, latent enthalpy effects, and chemical reaction velocity constant. Hie chapter concludes with a section on praperty estimation. 

Latent enthalpy effects can be measured directly in a calorimeter for vapor-liquid changes. The notation employed is AH with units of energy per mole, e.g., Btu/lbmol lower case h is used if based on mass, e.g., cal/g. However, these effects can be predicted from semi-theoretical equations provided in the liteiature. ... 

Baxter (B3) uses an enthalpy-flow temperature method, due originally to Dusinberre (D5, D6) and Eyres et al. (E4), whereby the movingboundary effect is reduced to a property variation. To begin with, the melting of a slab of finite thickness initially at the fusion temperature is considered. At the surface of the melt, which is of the same density as the solid, a heat transfer boundary condition is applied. The technique takes into account latent heat effects by allowing the specific heat to become infinite at the fusion temperature in such a way that... 

The term phase for a pure substance refers to a state of matter that is gas, liquid, or solid. Latent enthalpy (heat) effects are associated with phase changes. These phase changes involve no change in temperature but there is a transfer of energy to/from the substance. There are three possible latent effects, as detailed below ... 

The other class of formulations of the FEM is based on the definition of an effective specific heat. This results in the inclusion of the latent heat effect in the capacitance matrix. There are a number of ways in which this can be provided for. Each of these methods makes use of an enthalpy temperature curve, for example. 

Single-Effect Evaporators The heat requirements of a singleeffect continuous evaporator can be calculated by the usual methods of stoichiometry. If enthalpy data or specific heat and heat-of-solution data are not available, the heat requirement can be estimated as the sum of the heat needed to raise the feed from feed to product temperature and the heat required to evaporate the water. The latent heat of water is taken at the vapor-head pressure instead of at the product temperature in order to compensate partiaUv for any heat of solution. If sufficient vapor-pressure data are available for the solution, methods are available to calculate the true latent heat from the slope of the Diihriugliue [Othmer, Ind. Eng. Chem., 32, 841 (1940)]. 

It can be seen from Figure 13.5 that for the air-water system a straight line, of slope equal to the enthalpy of dry saturated steam (2675 kJ/kg), is almost parallel to the isothermals. so that the addition of live steam has only a small effect on the temperature of the gas. The addition of water spray, even if the water is considerably above the temperature of the gas, results in a lowering of the temperature after the water has evaporated. This arises because the. latent heat of vaporisation of the liquid constitutes the major part of the enthalpy of the vapour. Thus, when steam is added, it gives up a small amount of sensible heat to the gas, whereas when hot liquid is added a small amount of sensible heat is given up and a very much larger amount of latent heat is absorbed from the gas. 

W-3 CHF correlation. The insight into CHF mechanism obtained from visual observations and from macroscopic analyses of the individual effect of p, G, and X revealed that the local p-G-X effects are coupled in affecting the flow pattern and thence the CHF. The system pressure determines the saturation temperature and its associated thermal properties. Coupled with local enthalpy, it provides the local subcooling for bubble condensation or the latent heat (Hfg) for bubble formation. The saturation properties (viscosity and surface tension) affect the bubble size, bubble buoyancy, and the local void fraction distribution in a flow pattern. The local enthalpy couples with mass flux at a certain pressure determines the void slip ratio and coolant mixing. They, in turn, affect the bubble-layer thickness in a low-enthalpy bubbly flow or the liquid droplet entrainment in a high-enthalpy annular flow. 

Because the latent heat A//vap(r, P) is nearly constant over the usual effective range of T and P variations, we assume that it can be replaced by thz fixed molar enthalpy of vaporization AH°wap for standard state conditions (e.g., 25°C, 1 atm) ... 

The enthalpy conservation equation of the multiphase medium, obtained from the sum of the appropriate balance equations of the constituents includes the heat effects due to phase changes and hydration (dehydration) process, as well as the convectional and latent heat transfer,... 

Among other things, it became established that the nature of the structure adopted by a given compound on crystallization would then exert a profound effect on the solid-state properties of that system. For a given material, the heat capacity, conductivity, volume, density, viscosity, surface tension, diffusivity, crystal hardness, crystal shape and color, refractive index, electrolytic conductivity, melting or sublimation properties, latent heat of fusion, heat of solution, solubility, dissolution rate, enthalpy of transitions, phase diagrams, stability, hygroscopicity, and rates of reactions were all affected by the nature of the crystal structure. 

In this expression, q u is calculated with the correlation of choice, while hiG is the latent heat of vaporization and hsub in the enthalpy change necessary to bring the incoming subcooled liquid to saturation. Thus, from a design point of view, once the critical heat flux is known, the maximum exit vapor quality to avoid CHF can be calculated. It should be pointed out that Xcrit is not often the same value as x, which is the onset of dryout, since the latter can occur from a hydrodynamic effect (vapor shear) at low heat flux. 

ENTHALPY BALANCES FOR SINGLE-EFFECT EVAPORATOR. In a single-effect evaporator the latent heat of condensation of the steam is transferred through a heating surface to vaporize water from a boiling solution. Two enthalpy balances are needed, one for the steam and one for the vapor or liquid side. 

The liquor-side enthalpies // , Hf, and H depend upon the characteristics of the solution being concentrated. Most solutions when mixed or diluted at constant temperature do not give much heat effect. This is true of solutions of organic substances and of moderately concentrated solutions of many inorganic substances. Thus sugar, salt, and papermill liquors do not possess appreciable heats of dilution or mixing. Sulfuric add, sodium hydroxide, and caldum chloride, on the other hand, especially in concentrated solutions, evolve considerable heat when diluted and so possess appreciable heats of dilution. An equivalent amount of heat is required, in addition to the latent heat of vaporization, when dilute solutions of these substances are concentrated to high densities. 

Ideal gas enthalpy, Hfv> is obtained from (4-60). The derivative of pure component liquid fugacity coefBcient with respect to temperature leads to the following relation for combined effects of pressure and latent heat of phase change from vapor to liquid. 

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