Single-Component Condenser

Evaporation processes usually separate a single component (typically water) from a nonvolatile material. As such, it is good enough in most cases to assume that the vaporization and condensation processes take place at constant temperatures. 

This process has been used for various situations (1—14). Eor the condensation of a single component from a binary gas mixture, the gas-stream sensible heat and mass-transfer equations for a differential condenser section take the following forms ... 

Shell-Side Arrangements The one-pass shell (Fig. 11-35E) is the most commonly used arrangement. Condensers from single component vapors often have the nozzles moved to the center of the shell for vacuum and steam sei vices. 

To compare the performance between single component refrigerants and blends it will be necessary to specify the evaporating temperature of the blend to point A on the diagram and the condensing temperature to point B. 

The correlations given in the previous sections apply to the condensation of a single component such as an essentially pure overhead product from a distillation column. The design of a condenser for a mixture of vapours is a more difficult (ask. 

The term dT/dHt can be evaluated from the condensation curve h c from the single component correlations and h g from correlations for forced convection. 

When the fluid being vaporised is a single component and the heating medium is steam (or another condensing vapour), both shell and tubes side processes will be isothermal and the mean temperature difference will be simply the difference between the saturation temperatures. If one side is not isothermal the logarithmic mean temperature difference should be used. If the temperature varies on both sides, the logarithmic temperature difference must be corrected for departures from true cross- or counter-current flow (see Section 12.6). 

This section describes the phase change process for a single component on a molecular level, with both vaporization and condensation occurring simultaneously. Molecules escape from the liquid surface and enter the bulk vapor phase, whereas other molecules leave the bulk vapor phase by becoming attached to the liquid surface. Analytical expressions are developed for the absolute rates of condensation and vaporization in one-component systems. The net rate of phase change, which is defined as the difference between the absolute rates of vaporization and condensation, represents the rate of mass... 

This chapter introduces additional central concepts of thermodynamics and gives an overview of the formal methods that are used to describe single-component systems. The thermodynamic relationships between different phases of a single-component system are described and the basics of phase transitions and phase diagrams are discussed. Formal mathematical descriptions of the properties of ideal and real gases are given in the second part of the chapter, while the last part is devoted to the thermodynamic description of condensed phases. 

The thermodynamic properties of single-component condensed phases are traditionally given in tabulated form in large data monographs. Separate tables are given for each solid phase as well as for the liquid and for the gas. In recent years analytical representations have been increasingly used to ease the implementation of the data in computations. These polynomial representations typically describe the thermodynamic properties above room temperature (or 200 K) only. 

The basic assumptions implied in the homogeneous model, which is most frequently applied to single-component two-phase flow at high velocities (with annular and mist flow-patterns) are that (a) the velocities of the two phases are equal (b) if vaporization or condensation occurs, physical equilibrium is approached at all points and (c) a single-phase friction factor can be applied to the mixture if the Reynolds number is properly defined. The first assumption is true only if the bulk of the liquid is present as a dispersed spray. The second assumption (which is also implied in the Lockhart-Martinelli and Chenoweth-Martin models) seems to be reasonably justified from the very limited evidence available. 

The results obtained at surface pressures of 5 and 10 dynes per cm. are complicated by the fact that the trilaurin has undergone a two-dimensional condensation at 5°C., and we will not attempt interpretation at this time. A complete understanding of the molecular interactions involved in such mixed films must await a deeper insight into the interactions involved in films of pure components. We are presently attempting to evaluate such interactions by a thermodynamic study of compression of single-component films (5), with the ultimate objective of obtaining a fuller understanding of mixed films on a molecular basis. 

Single-Component System with Isotropic Interfaces and No Strain Energy. This relatively simple case could, for example, correspond to the nucleation of a pure solid in a liquid during solidification. For steady-state nucleation, Eq. 19.16 applies with AQC given by Eq. 19.4 and it is necessary only to develop an expression for /3C. In a condensed system, atoms generally must execute a thermally activated jump over a... 

In the figure 3 we present data on single component adsorption isotherms and simulation results. Data obtained from the literature [6] are included for comparison. The increase of the loading observed on the high pressures region can be explained by capillaiy condensation in the exterior secondary pore system, in particular between the crystals. This has been observed also by other authors [S]. 

Consider now what happens if the liquid is a mixture of several components. As heat is added, the liquid temperature rises until a temperature is reached at which the first bubble of vapor forms. Up to this point, the process looks like that for a single component. However, if the liquid is a mixture, the vapor generated generally will have a composition different from that of the liquid. As vaporization proceeds, the composition of the remaining liquid continuously changes, and hence so does its vaporization temperature. A similar phenomenon occurs if a mixture of vapors is subjected to a condensation process at constant pressure at some temperature the first droplet of liquid forms, and thereafter the composition of the vapor and the condensation temperature both change. 

If a liquid mixture is heated above its bubble point, the vapor generated is rich in the more volatile mixture components. As vaporization continues, the system temperature steadily increases (unlike the case for a single-component system, in which T remains constant). Similarly, if a vapor mixture is cooled below its dew point, the liquid that condenses is rich in the less volatile components and the temperature progressively decreases. 

---