Effect of Liquid Viscosity on Liquid Film Mass Transfer Coefficient for Structured and Random Packings

Structured and high performance random packings are commonly used in distillation, absorption and stripping applications. While gas film mass transfer coefficient models often show a consistent form, the form of liquid film mass transfer coefficient models (k_L) vary considerably especially in regards to the dependence on liquid viscosity. A good prediction of liquid film mass transfer coefficient (k_L) is important for an efficient design of packed columns. Despite the large numbers of existing k_L correlations, most of them only investigated water with little variance of liquid viscosity. Extrapolation of these correlations to separation process of viscous liquid may thus be problematic.

This work investigated the effect of liquid viscosity on k_L by air stripping of toluene from aqueous solutions of glycerol in a semi-works scale (0.43 m I.D.) packed column. The liquid viscosity was varied 1–60 cP by varying the glycerol concentration 0–89 wt %. Packings with different specific area, corrugation angle, and special corrugation channel design were investigated. A total of nine packings were studied, which include seven structured packings (Mellapak 125Y, Mellapak 250X, Mellapak 250Y, GTC GT-OPTIMPAK 250Y, GTC GT-PAK 350Y, GTC GT-PAK 500Y and Montz B1 250 MN,) and two random packings (Raschig Super Rings # 0.5 and # 1.5).

Based on a previous study that showed liquid viscosity has insignificant effect of mass transfer area, a new k_L model was developed that showed the total dependence on liquid viscosity of k_L was -0.89, in which -0.35 was the indirect dependence via diffusivity, and the rest was the direct dependence through liquid turbulence on the packing surface. Liquid viscosity has a similar effect on structured and random packings. Compared to existing k_L correlations, this work utilizes a large database of packings, a wider range of liquid viscosity, reliable experimental data of mass transfer area, and relatively large size of column.