Dehumidification process

Figure 4 shows the general arrangement and nomenclature for a humidification or dehumidification process, where the subscript 1 refers to the bottom of the column, and subscript 2 to the top. Steady state is assumed. Flow rates and compositions are given in molar terms because this simplifies the results. 

Fig. 4. Arrangement and nomenclature for general humidification—dehumidification process.

Figure 12-8.3. Schematic drawing of (a) basic sorption dehumidification process and (b) compressor-assisted air dehumidification process.

In these multifunctional processes, heat transfer and mass transfer are two combined and simultaneous functions, and the objective is to substantially improve these functions in order to save energy, to increase the process efficiency, and to reduce the size and cost of industrial plants. Corrugated pads are often used in the dehumidification process or in chemical heat pumps, but a higher efficiency could be reached by using diabatic units, where the wall could exchange heat with the liquid film. 

Flat-membrane contactors have been specifically designed and commercialized by GVS SpA (Italy) for air dehumidification processes [23]. 

Define the dry-bulb temperature, wet-bulb temperature, and humid volume of humid air. Given values of any two of the variables plotted on the psychrometric chart (dry-buib and wet-bulb temperatures, absolute and relative humidity, dew point, humid volume), determine the remaining variable values and the specific enthalpy of the humid air. Use the psychrometric chart to carry out material and energy balance calculations on a heating, cooling, humidification, or dehumidification process involving air and water at 1 atm. 

Wagner B, Jehle W, Steinwandel J, Staneff T, and Herczog J, Continuous membrane supported cabin air dehumidification process. Euromembrane 97, University of Twente, Enschede, The Netherlands, June 26, 1997. 

Bergero S and Chiari A. Experimental and theoretical analysis of air humidification/dehumidification processes using hydrophobic capillary contactors. Appl. Then Eng. 2001 21 1119-1135. 

Sometimes the dehumidification process can lead to fog formation. Consider a gas with the initial conditions represented by point A in Fig. 23.7. In this case the dehumidification path from A to B reaches the equilibrium curve and then crosses it into a region of supersaturation. The rate of heat transfer (temperature change) outruns the mass-transfer rate (humidity change) so that the gas becomes supersaturated. If the gas contains dust or other particles that can serve as nuclei for droplet formation, the supersaturation may be relieved by condensation on these particles instead of on the bulk liquid surface. This can lead to a persistent, troublesome fog. Fog formation may be avoided by making sure that the initial gas temperature is well above the equilibrium value, as at point A, or by adding heat to the gas during the dehumidification process. ... 

Dchumidificatlon by other methods Adsorption, e.g., methods using activated silica gel, alumina, or molecular sieves as adsorbents (see Chap. 11), and washing gases with water solutions containing dissolved substances which appreciably lower the partial pressure of the water (see Chap. 8) are other commonly used dehumidification processes, particularly when very dry gases are required. 

Membrane dehumidification method is advantageous over the conventional methods since the running cost is low, the construction of the apparatus is simple, and the dry gas can be produced continuously. The key to membrane driven dehumidification process is to provide a moisture removing membrane having both high water vapour permeation rate and high selectivity of water over other gases [24]. 

---