**3. Conclusions**

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 random and structured packing.

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 calculation of areas and HETP.

For the distillation in packed columns, it was ascertained that the resistance in both phases, liquid and vapor phases, should be taken into account in the HETP evaluation.

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 nominal sizes of the packing.

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 system studied.

Finally, to better evaluate HETP, it is also important to choose a thermodynamic model that can represent the behavior of the liquid-vapor equilibrium and complex methodologies to calculate the theoretical number of stages.
