Transport Characteristics of Mellapak 250Y Packing Under Distillation Conditions. Effect of Axial Mixing of the Phases

Transport characteristics (HETP, k_La , k_Ga) of metal Mellapak 250Y structured packing has been measured in the distillation column distilling three alcoholic systems (methanol-ethanol, ethanol-propanol, methanol-propanol) and one standard hydrocarbon system (cyclohexane-heptane) under total reflux condition at atmospheric pressure. Volumetric mass-transfer coefficients have been determined using "profile method".

Experimental aparatus was an atmospheric distillation column of an inner diameter of 0,15m packed with ten pieces of packing with the total height of the packing 2,1m. The column is equiped with seven sampling points along the packing enabling withdrawal of a sample of liquid and vapour directly from the packing and therefore measurement of concentration profiles in both phases. Experiments were performed for four powers into a reboiler up to u_G = 1,7m/s.

An axial mixing in terms of Bodenstein numbers Bo_L, Bo_G has been measured independently in the absorption column with water-air system. There has been found that axial mixing data transfer from absorption to distillation can be succesfully achieved using relation suggested by Fair (1999). Volumetric mass-transfer coefficients evaluated with axial mixing being taken into account differ substantially from those evaluated with common plug-flow assumption. Results indicate significant mass-transfer resistance in both phases and significant influence of axial mixing of the liquid phase on separation process. The effect of gas phase axial mixing was found negligible. Dependence of volumetric mass-transfer coefficients on the phase flow rates differs from the ones incorporated in commonly used models by Olujic', Billet-Schultes and Rocha-Bravo-Fair.