Latent heat molal

Figure 28 shows the key features of the humidity chart. The chart consists of the following four parameters plotted as ordinates against temperature on the abscissas (1) Humidity H, as pounds of water per pound of dry air, for air of various relative humidities (2) Specific volume, as cubic feet of dry air per pound of dry air (3) Saturated volume in units of cubic feet of saturated mixture per pound of dry air and (4) latent heat of vaporization (r) in units of Btu per pound of water vaporized. The chart also shows plotted hiunid heat (s) as abscissa versus the humidity (H) as ordinates, and adiabatic humidification curves (i.e., humidity versus temperature). Figure 28 represents mixtures of dry air and water vapor, whereby the total pressure of the mixture is taken as normal barometric. Defining the actual pressure of the water vapor in the mixture as p (in units of mm of mercury), the pressure of the dry air is simply 760 - p. The molal ratio of water vapor to air is p/(760-p), and hence the mass ratio is ... 

Estimate the molal latent heat of vaporization for pure benzene at its normal boiling point of 80.1°C. 

The integral terms representing AH and AH can be computed if molal heat capacity data Cp(T) are available for each of the reactants (i) and products (j). When phase transitions occur between T and Tj for any of the species, proper accounting must be made by including the appropriate latent heats of phase transformations for those species in the evaluation of AHj, and AH terms. In the absence of phase changes, let Cp(T) = a + bT + cT describe the variation of (cal/g-mole °K) with absolute temperature T (°K). Assuming that constants a, b, and c are known for each species involved in the reaction, we can write... 

The cost of refrigerant is 11.5 GJ 1, the mean molal heat capacity of the vapor is 40 kJ kmol-1K-1, and the latent heat of acetone is 29,100 kJ kmol-1. 

Hv/Te Molal latent heat of vaporization at te divided by Te. (Equal to the molal entropy of vaporization at te.)... 

Mt is lhe molar mass of the solvent, AJin its latent heat of vaporization in kcal per mole at temperature 7 . ms the molality of solute v 0e is called... 

The raising of the boiling point and the lowering of the freezing point are therefore proportional to the molal concentration. The constant of proportionality can be calculated from the latent heat of evaporation or fusion, and from the boiling or freezing point of the solvent. 

A = difference between exit and entering stream also used for final minus initial times or small time increments A = molal heat of vaporization A/ = molal latent heat of fusion at the melting point p = density... 

These stagewise calculations are nesijy programmed for computer solution, It the calculations are made by hand, U may not be necessary to ran an energy balance at every stage, unless there are large differences between die molal latent heats of vaporization of the mixture components. At any rale, if the equilibrium and enthalpy data are at hand, the computations proceed quite rapidly, even if done hy hand. 

HEATING AND COOLING REQUIREMENTS. Heat loss from a large insulated column is relatively small, and the column itself is essentially adiabatic. The heat effects of the entire unit are confined to the condenser and the reboiler. If the average molal latent heat is X and the total sensible heat change in the liquid streams is small, the heat added in the reboiler is VX, either in watts or Btu per hour. When the feed is liquid at the bubble point (q 1), the heat supplied in the reboiler is approximately equal to that removed in the condenser, but for other values of q this is not true. (See page 554.)... 

V = vapor rate from reboiler Aj = latent heat of steam X = molal latent heat of mixture... 

The boiling point-equilibrium data for the system acetone-methanol at 760 mm Hg are given in Table 18.7. A column is to be designed to separate a feed analyzing 25 mole percent acetone and 75 mole percent methanol into an overhead product containing 78 mole percent acetone and a bottom product containing 1.0 mole percent acetone. The feed enters as an equilibrium mixture of 30 percent liquid and 70 percent vapor. A reflux ratio equal to twice the minimum is to be used. An external reboiler is to be used. Bottom product is removed from the reboiler. The condensate (reflux and overhead product) leaves the condenser at 25°C, and the reflux enters the column at this temperature. The molal latent heats of both components are 7700 g cai/g mol. The Murphree plate efficiency is 70 percent. Calculate (a) the number of plates required above and below the feed (b) the heat required at the reboiler, in Btu per pound mole of overhead product (c) the heat removed in the condenser, in Btu per pound mole of overhead product. 

HUMIDITY CHARTS FOR SYSTEMS OTHER THAN AIR-WATER. A humidity chart may be constructed for any system at any desired total pressure. The data required are the vapor pressure and latent heat of vaporization of the condensable component as a function of temperature, the specific heats of pure gas and vapor, and the molecular weights of both components. If a chart on a mole basis is desired, all equations can easily be modified to the use of molal units. If a chart at a pressure other than 1 atm is wanted, obvious modificatioi in the above equations may be made. Charts for several common systems besides air-water have been published. ... 

Then, every mole of condensing vapor vaporizes exactly 1 mole of liquid. Since it is the molar latent heats that are presumed to be equal, the flows must be specified in terms of moles, and the concentrations in terms of mole fractions for use with the constant molal overflow assumption. 

An insulated glass tube and condenser are mounted on a reboiler containir benzene and toluene. The condenser returns liquid reflux in such a manner that it runs down the wall of the tube. At one point in the tube the temperature is 170°F, the vapor contains 30mole% toluene, and the liquid reflux contains 40mole% toluene. The effective thickness of the stagnant vapor film is calculated to be 0.1 in. The molal latent heats of benzene and toluene are equal. Calculate the rate at which toluene and benzene are being interchanged at this point in the tube in pound moles per hour per square foot. 

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