**4. Nomenclature**

62 Mass Transfer in Chemical Engineering Processes

The use of packed columns for continuous contacting of vapors and liquids is well established in the chemical industry, nowadays. The design of the columns require a knowledge of the height of a transfer unit and this chapter had as main objective the description of the present correlations, relating their advantages and disadvantages, for

Among the researches encountered in the literature and cited in this chapter, it is important to have a model that describes the fluid dynamic relationships in packed columns with countercurrent flow of the gas and liquid phases to describe up the flood point. It is so important because above this point, the liquid accumulates to such an extent that column instability occurs. The disadvantage of some correlations relies on the fact that many parameter characteristics is only obtained graphically, what introduces deviations in the

For the distillation in packed columns, it was ascertained that the resistance in both phases,

About the packing, new random and structured packing have been studied, but the difficulty in HETP representation remains the problem, due to the fact that it is so difficult to find a correlation that covers all systems with different physical properties and different

Moreover, normally, HETP is substantially constant over a wide range of vapor flows; on the other hand, vapor flow varies increasing or decreasing the mass transfer depending on the liquid phase. Because of that, HETP is not constant along the column and it is convenient to define one value that which may be used for design purposes. Due to these factors, the correlations proposed, empirical or theoretical, do not reach the real value of HETP for any

liquid and vapor phases, should be taken into account in the HETP evaluation.

Fig. 7. The theoretical plates of PACK-13C (Li et al., 2010)

**3. Conclusions** 

random and structured packing.

calculation of areas and HETP.

nominal sizes of the packing.

system studied.

*dP* – nominal size of the packing


*ρ* – density

*H* – height representation of the mass transfer unit


h – operating holdup - m3/m3


HETP Evaluation of Structured and Randomic Packing Distillation Column 65

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*SCV -* Schmidt number of the vapor phase


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$$R\_{eL} = \frac{4\delta\mu\_{L\epsilon}\rho\_L}{\mu\_L} \tag{\text{Reynolds number for liquid}}$$

$$F\_{rL} = \frac{\mu\_L^2}{S\text{g}} \tag{\text{Froude number for liquid}}$$

$$\mathcal{W}\_{eL} = \frac{\mu\_L^2 \rho\_L S}{\sigma \mathcal{g}\_c} \tag{\text{Weber number for liquid}}$$

### **5. References**


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> 2 *L L*

**5. References** 

*u S <sup>W</sup> g* 

*L*

*c*

*eL*

*rL <sup>u</sup> <sup>F</sup>*

*eL*

*<sup>u</sup> <sup>R</sup>* 

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*D* – diffusivity


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**4** 

*Brazil* 

**Mathematical Modelling of Air** 

*1Centro Federal de Educação Tecnológica, CEFET-Rio 2Universidade Federal do Rio de Janeiro, COPPE/UFRJ* 

**Drying by Adiabatic Adsorption** 

Carlos Eduardo L. Nóbrega1 and Nisio Carvalho L. Brum2

The careful control of ambient air moisture content is of concern in many industrial processes, with diverse applications such as in metallurgical processes or pharmaceutical production. In the air-conditioning field, the increasingly concern with sick building syndrome also brings humidity control into a new perspective. Underestimated ventilation rates might result in poor indoor air quality, with a high concentration of volatile organic compounds, smoke, bacteria and other contaminants. Epidemiological studies indicate a direct connection between inadequate levels of moisture and the incidence of allergies and infectious respiratory diseases. A popular method of lowering the concentration of contaminants is to increase the ventilation rates. In fact, the fresh air requirement per occupant/hour imposed by the current air-quality standard has doubled over the last three decades. Since the fresh air has to be brought to the thermal comfort condition, increased ventilation rates imply increased thermal loads, which in turn will demand chillers with increased cooling capacity. Accordingly, there is a trade-off between indoor air quality and

energy consumption, which is also of main concern of private and public sectors.

cooling systems exclusively employ water as the refrigerant.

applications on low humidity areas (Khuen et al., 1998).

Figure (1.a) shows an evaporative cooling system. It essentially consists of a chamber through which air is forced through a water shower. It is a sound system from air-quality, energy consumption and ecological viewpoints. The air quality is provided by a continuous air room change, with no air recirculation. Since the cooling effect is provided by evaporation of water into air, the energy consumption is restricted to the pumping power, which is usually low when compared to the energy needs of a compressor. Unlike vaporcompression systems, which usually employ ozone-depleting refrigerants, evaporative

Figure (1.b) shows that the evaporative cooling process is isenthalpic, which means that the air stream enthalpy remains unaltered as it flows through the evaporative cooler. Accordingly, the increase in the air stream humidity occurs at the expense of its own sensible energy, and the air stream is cooled and humidified as it crosses the evaporative cooler. Since the heat and mass transfer processes are mutually dependent, the air stream humidity at the inlet of the evaporative cooler has to be significantly low, if an appreciable cooling effect is to be achieved. Unfortunately this is not always the case, and this cooling technique is not as effective as traditional vapour-compression systems, being restricted to

**1. Introduction**

