Condensing Vapors and Boiling Liquids

Heating by condensing vapors, usually by saturated steam, is a very common practice in chemical and bioprocess plants. Liquid boiling and vapor condensation also occur in distillation or evaporation equipment. 

To provide some examples, h-values for the film-type condensation of water vapor (when a film of condensed water would cover the entire cooling surface) will range from 4000 to 15 OOOkcal h-1 m-2 °C 1, while those for boiling water would be in the range of 1500 to 30 000 kcal h 1 m 2 oC 1. 

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The net effect of dividing the feed stream into nearly equal portions is to elongate the T-Q diagram as shown in Figure 10.43b. Note that the total area is not conserved because the pressure level in each effect determines the bubble-point temperatures of its condensing vapor and boiling liquid streams. 

The addition of effects also eventually runs into economic limitations, even at high energy costs. If the initial steam pressures and final concentrator pressure are set by other processing considerations, the allowable AT between condensing vapor and boiling liquid decreases in proportion to the number of effects. As the average AT decreases, the surface and therefore capital cost increases. This economic balance then must be met. 

Flash distillation is used most for separating components that boil at widely different temperatures. It is not effective in separating components of comparable volatility, since then both the condensed vapor and residual liquid are far from pure. By many successive redistillations small amounts of some nearly pure components may finally be obtained, but this method is too inefficient for industrial distillations when nearly pure components are wanted. Methods now used in both laboratory and plant apply the principle of rectification, which is described in this section. 

Vapor pressure is a measure of the tendency of a substance to be in the gas phase at a given temperature. At the boiling point of a liquid mixture, the component with the higher vapor pressure will have a higher concentration in the condensing vapors and thus will be purer than it was before. 

Explain in your own words the terms bubble point, boiling point, and dew point of a mixture of condensable species, and the difference between vaporization and boiling. Use Raoult s law to determine (a) the bubble-point temperature (or pressure) of a liquid mixture of known composition at a specified pressure (or temperature) and the composition of the first bubble... 

Consider transport across the phase boundary shown in Figure 11.3. We shall denote the two bulk phases by L and V and the interface by I. Though the analysis below is developed for liquid-vapor interphase transport the formalism is generally valid for all two-phase systems. Therefore, what follows applies equally to distillation, stripping, and absorption operations. With a few modifications (to be described later), the analysis below may be used in the determination of rates of condensation, evaporation, vaporization, and boiling. 

Distillation is a method of separation based on the difference in composition between a liquid mixture and the vapor formed from it. The composition difference is due to differing effective vapor pressures, or volatilities, of the components of the liquid. When such a difference does not exist, as at an azeotropic point, separation by distillation is not possible. The most elementaiy form of the method is simpledistillation in which the liquid mixture is brought to boiling and the vapor formed is separated and condensed to form a product if the process is continuous, it is called flash distillation or an equilibrium flash, and if the feed mixture is available as an isolated batch of maleiial. the process is a form of batch distillation and the compositions of the collected vapor and residua] liquid are thus time dependent. 

Processes of heat transfer accompanied by phase change are more complex than simple heat exchange between fluids. A phase change involves the addition or subtraction of considerable quantities of heat at constant or nearly constant temperature. The rate of phase change may be governed by the rate of heat transfer, but it is often influenced by the rate of nucleation of bubbles, drops, or crystals and by the behavior of the new phase after it is formed. This chapter covers condensation of vapors and boiling of liquids. Crystallization is discussed in Chap. 27. 

Many continuous extractions involving solid samples are carried out with a Soxhiet extractor (Figure 7.18). The extracting solvent is placed in the lower reservoir and heated to its boiling point. Solvent in the vapor phase moves upward through the tube on the left side of the apparatus to the condenser where it condenses back to the liquid state. The solvent then passes through the sample, which is held in a porous cellulose filter thimble, collecting in the upper reservoir. When the volume of solvent in the upper reservoir reaches the upper bend of the return tube, the solvent and any extracted components are siphoned back to the lower reservoir. Over time, the concentration of the extracted component in the lower reservoir increases. 

Recirculation of non-boiling liquids can be achieved by bubbling inert gas through the liquid in the reactor jacket. This is less practical for fluids with significant vapor pressure, because the jacket still must be under pressure, and a large condenser must be installed to condense the liquid from the vapor-saturated gas at the jacket temperature. It is more useful with molten metals and salts. For the design details of the reactor tube s inside, the same considerations apply as for a thermosiphon-controlled reactor. 

The term still is applied only to the vessel in which liquids are boiled during distillation, but the term is sometimes applied to the entire apparatus, including the fractionating column, the condenser, and the receiver in which the distillate is collected. If a water and alcohol distillate is returned from the condenser and made to drip down through a long column onto a series of plates, and if the vapor, as it rises to the condenser, is made to bubble through this liquid at each plate, the vapor and liquid will interact so that some of the water in the vapor condenses and some of the alcohol in the liquid vaporizes. The interaction at each plate is equivalent to a redistillation. This process is referred to by several names in the industry namely rectification, fractionation, or fractional distillation. 

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