Heat of evaporation, 2.20

The first effect is illustrated when we blow across a bowl of hot soup, to cool the soup. Our breath displaces the steam vapors that are on top of the soup. This encourages more molecules of steam vapors to escape from the soup that is, the vapor pressure of the steam above the liquid soup is diminished, because steam is pushed out of the soup bowl with air. The correct technical way to express this idea is to say, The partial pressure of the steam, in equilibrium with the soup, is diminished. But our breath itself does not remove heat from the soup. The evaporation of steam from the soup, promoted by our breath, takes heat. Converting one pound of soup to one pound of steam requires 1000 Btu. This heat of evaporation comes not from our breath, but from the soup itself. The correct technical way to express this second effect is, The sensible-heat content of the soup is converted to latent heat of evaporation.  

For example, if we have 101 lb of soup in a rather large bowl, and cause one pound to evaporate by blowing across the bowl, the soup will lose 1000 Btu. This heat of evaporation will come at the expense of the temperature of the remaining soup in the bowl that is, each pound of soup will lose 10 Btu. If the specific heat of our soup is 1.0 But/[(lb)(°F)], the soup will cool off by 10°F. 

A steam stripper, as shown in Fig. 10.1, works in the same way. The diesel-oil product drawn from the fractionator column is contaminated with gasoline. The stripping steam mixes with the diesel-oil product on the trays inside the stripper tower. The steam reduces the hydrocarbon partial pressure and thus allows more gasoline to vaporize and to escape from the liquid phase into the vapor phase. The heat of vaporization of the gasoline cannot come from the steam, because the steam (at 300°F) is colder than the diesel oil (at 500°F). The heat of vaporization must come from the diesel-oil product itself. 

We can use this idea to calculate the percent of diesel oil that would actually vaporize across the stripping trays in the stripping tower. Let s assume the following thermal properties for a typical hydrocarbon mixture of diesel and gasoline  

Referring to Fig. 10.1, the reduction in sensible heat of the diesel product equals  

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Acryhc and methacryhc nonaqueous dispersions (NADs) are primarily utilized by the coatings industry to avoid certain difficulties associated with aqueous dispersion (emulsion) polymers. Water as a suspension medium has numerous practical advantages, but also some inherent difficulties a high heat of evaporation, a low boiling point, and an evaporation rate that depends on the prevailing humidity. Nonaqueous dispersions alleviate these problems, but introduce others such as flammabihty, increased cost, odor, and toxicity. 

Modules Eveiy module design used in other membrane operations has been tried in peivaporation. One unique requirement is for low hydraulic resistance on the permeate side, since permeate pressure is veiy low (O.I-I Pa). The rule for near-vacuum operation is the bigger the channel, the better the transport. Another unique need is for neat input. The heat of evaporation comes from the liquid, and intermediate heating is usually necessary. Of course economy is always a factor. Plate-and-frame construc tion was the first to be used in large installations, and it continues to be quite important. Some smaller plants use spiral-wound modules, and some membranes can be made as capiUaiy bundles. The capillaiy device with the feed on... 

Finally, it is to be expected that the evaporation coefficient of a very stable compound, such as alumina, which has a large heat of sublimation resulting from the decomposition into the elements, will be low. Since the heat of evaporation must be drawn from the surface, in die case of a substance widr a low thermal conductivity such as an oxide, the resultant cooling of the surface may lead to a temperature gradient in and immediately below the surface. This will lower die evaporation rate compared to that which is calculated from the apparent, bulk, temperature of the evaporating sample as observed by optical pyromeuy, and thus lead to an apparently low free surface vaporization coefficient. This is probably die case in the evaporation of alumina in a vacuum. 

Condensation is the process of reduction of matter into a denser form, as in the liquefaction of vapor or steam. Condensation is the result of the reduction of temperature by the removal of the latent heat of evaporation. The removal of heat shrinks the volume of the vapor and decreases the velocity of, and the distance between, molecules. The process can also be thought of as a reaction involving the union of atoms in molecules. The process often leads to the elimination of a simple molecule to form a new and more complex compound. 

L, = latent heat of evaporation of steam at flash pressure, Btu/lb... 

T = temperature of tube heating surface, °R Tj = saturation temperature of liquid, °R Bl = coefficient of Figure 10-99 hfg = latent heat of evaporation, Btu/lb... 

A recent development in heat recovery has been the heat tube. This is a sealed metal tube which has been evacuated of air and contains a small quantity of liquid which, for boiler applications, could be water. When heat from the flue gases is applied to one end of the heat pipes the water in the tube boils, turning to steam and absorbing the latent heat of evaporation. The steam travels to the opposite end of the tube which is surrounded by water, where it gives up its latent heat, condenses and returns to the heated end of the tube. Batteries of these tubes can be arranged to form units, usually as a water jacket around a section of a flue. 

This is the device where the air or water being cooled gives up its heat to provide the latent heat of evaporation to the refrigerant. Superheat is also added to the refrigerant at this point to prevent damaging liquid forming on the way to the compressor. 

Water is injected into the air stream in a fine mist by pumped jets or spinning disc. For practical purposes, the psychrometric plot follows a wet bulb line. The air provides the latent heat of evaporation, resulting in a fall in dry bulb temperature. If water were to be supplied at up to 100°C the humidified condition would be at a correspondingly higher total heat of 420 kJ per kg water supplied. 

A liquid boils and condenses - the change between the liquid and gaseous states - at a temperature which depends on its pressure, within the limits of its freezing point and critical temperature. In boiling it must obtain the latent heat of evaporation and in condensing the latent heat must be given up again. 

If by water spray or washer, the necessary heat must be put into the air first to provide the latent heat of evaporation. This can be done in two stages, A to T to C, or three stages A to Hto Jto C, if reheat is required to get the exact final temperature. The latter is easier to control. 

Moisture can be removed from any material which is to be dried, by passing air over it which has a lower water vapour pressure. Also, in removing this moisture, the latent heat of evaporation must be supplied, either directly by heating, or by taking sensible heat from the airstream which is carrying out the drying process. 

When r is not large compared with the length of either dipole the value will be somewhat greater. For two molecules in water we may substitute r = 2.9 X 10-8 centimeter, and for /j insert the value from Table 41. Since the observed heat of evaporation of water is in the neighborhood of 0.5 electron-volt, we expect to find a value of the order of 0.25 electron-volt, since we are discussing here a molecule that has only one neighbor. We obtain... 

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