Boiling Vaporization of Liquids

Boiling Mechanisms Vaporization of liquids may result from various mechanisms of heat transfer, singly or combinations thereof. For example, vaporization may occur as a result of heat absorbed, by radiation and convection, at the surface of a pool of liquid or as a result of heat absorbed by natural convection from a hot wall beneath the disengaging surface, in which case the vaporization takes place when the superheated liquid reaches the pool surface. Vaporization also occurs from falling films (the reverse of condensation) or from the flashing of liquids superheated by forced convection under pressure. 

Heat transfer by nucleate boiling is an important mechanism in the vaporization of liquids. It occurs in the vaporization of liquids in 

Boiling Coefficients For the nucleate-boiling coefficient the Mostinski equation [Teplenergetika, 4, 66 (1963)] may be used  

An equation of the Nusselt type has been suggested by Rohsenow [Trans. Am. Soc. Mech. Eng., 74, 969 (1952)]. 

The coefficient Cr is not truly constant but varies from 0.006 to 0.015. It is possible that the nature of the surface is partly responsible for the variation in the constant. The only factor in Eq. (5-94 ) not readily available is the value of the contact angle (V. 

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Heat transfer by nucleate boiling is an important mechanism in the vaporization of liqmds. It occurs in the vaporization of liquids in kettle-type and natural-circulation reboilers commonly usea in the process industries. High rates of heat transfer per unit of area (heat flux) are obtained as a result of bubble formation at the liquid-solid interface rather than from mechanical devices external to the heat exchanger. There are available several expressions from which reasonable values of the film coefficients may be obtained. 

P8.1 The molar enthalpy of vaporization of liquid mercury is 59.229 kJ-mol l at its normal boiling point of 630.0 K. The heat capacities of the liquid and gaseous phases, valid over the temperature range from 250 to 630 K, are as follows ... 

Self-Test 7.7A Use Trouton s rule to estimate the standard enthalpy of vaporization of liquid bromine, which boils at 59°C. 

FIGURE 11.19 A simple fractional distillation column used in a chemistry laboratory. The vapors from a boiling mixture of liquids rise inside the column, where they condense on contact with the cool column walls, drip back, and are reboiled by contact with more hot vapor. Numerous boil/condense cycles occur before vapors finally pass out the top of the column, reach the water-cooled condenser, and drip into the receiver. 

NH3 (liq.). The most accurate value for the heat of vaporization of liquid ammonia is that of Osborne and Van Dusen,1 who found V = —5.56 at the normal boiling point. From this value, we have computed V = — 5.07 at 18°. Other data on the heat of vaporization of ammonia are given by Estreicher and Schneer,1 Holst,1- 3 de Forcrand and Massol,1 Henning,1 Regnault,3 Strombeck,1 Franklin and Kraus,1 Brill,1 Eucken and Karwat,1 and Donath.1... 

HCOOH (g), (HCOOH)2 (g). Ramsperger and Porter2 and Coo-lidge1 studied the equilibrium, (HCOOH) 2 (g) =2HCOOH (g), and their data yield —14.13 for the heat of this reaction. The equilibrium concentrations in mole fraction are, for (HCOOH) 2 (g) and HCOOH (g), respectively, about 0.80 and 0.20 at 18°, and about 0.52 and 0.48 at the boiling point, 100.8°. Data on the heat of vaporization of liquid formic acid to form the equilibrium mixture of (HCOOH) 2 (g) and HCOOH (g)... 

The normal boiling point of liquid X is less than that of Y, which is less than that of Z. Which of the following is the correct order of increasing vapor pressure of the three liquids... 

Boiling Mechanisms Vaporization of liquids may result from various mechanisms of heat transfer, singly or combinations thereof... 

The vaporization process will be spontaneous at all temperatures at which AG° is negative. Note that AS° favors the vaporization process because of the increase in positional entropy, and AFP favors the opposite process, which is exothermic. These opposite tendencies will exactly balance at the boiling point of liquid Br2, since at this temperature liquid and gaseous Br2 are in equilibrium (AG° = 0). We can find this temperature by setting AG° = 0 in the equation... 

The normal boiling point of liquid ammonia is 240 K the enthalpy of vaporization at that temperature is 23.4 kj mol . The heat capacity of gaseous ammonia at constant pressure is 38 J moC ... 

The enthalpy of vaporization of liquid phosgene measured calorimetrically at the normal boiling temperature has been found to be 24.402 0.003 kJ mol" [751]. It thus takes about one-ninth as much heat to vaporize phosgene as would be required to vaporize the equivalent quantity of water. The less reliable value of H,... 

The very high purity of hydrogen from a liquid source arises from the fact that at the normal boiling point of liquid hydrogen, all materials (except helium) are frozen solid, have very low vapor pressures, and are essentially insoluble in the liquid hydrogen. Liquid hydrogen, therefore, when vaporized, is exceptionally pure if no recontamination has occurred. 

The boiling temperature of liquid L] is calculated on a trial and error basis. This must be lower than the temperature of the tower bottom. This boiling temperature is calculated on the basis that at a specific pressure this temperature must satisfy the relation Xy, = 1.0. For a mixture of components 1, 2, 3 and 4 with component 3 as the basis, Xy, = XK,x, = K3Xa,x, = X(Ps/P)x, = 1.0. The trial starts with a guess of a temperature. Then the values of ctj are calculated from the values of K, (read from a chart) at this temperature. The value of Kj is calculated as Kj = 1.0/Xa,x,. Then the temperature corresponding to the calculated value of K is compared to the assumed temperature. If the values are different, the calculated temperature is used for the next iteration. After the final temperature is known (i.e. convergence is reached), the vapor composition is calculated from equation 5.1 or its equivalents. 

The heat of vaporization of a liquid (A//yap) is the energy required to vaporize 1.00 g of the liquid at its boiling point. In one experiment, 60.0 g of liquid nitrogen (boiling point — 196°C) are poured into a Styrofoam cup containing 2.00 X 10 g of water at 55.3°C. Calculate the molar heat of vaporization of liquid nitrogen if the final temperature of the water is 41.0°C. 

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