(38c) An Interpretation of Packing HETP in a Pumparound

Pumparounds are often used in column designs, especially in heat transfer applications such as direct condensers and quenchers. In some instances, pumparounds are also used for mass transfer applications, even though recycling the liquid stream reduces concentration gradients and affects mass transfer driving forces negatively. There are several reasons for considering pumparounds in mass transfer applications, such as:

• (1) Systems with low thermodynamic k-value show a reduced concentration gradient deterioration but benefit from improved effective interfacial area due to a higher specific liquid load.
• (2) A pumparound with a sizeable liquid flow and a cooler can lower the temperature profile in the column, to give better thermodynamic behaviour.

A pumparound in an equilibrium stage simulator is simulated with at least two theoretical stages. If a pumparound cooler is present, more theoretical stages are usually required. Using the outcomes of the equilibrium stage simulation, the number of transfer units (NTU_OG) can be estimated. For a packed column, the height of a transfer unit (HTU_OG) can also be estimated using mass transfer correlations, and consequently the height of an equilibrium stage (HETP) can be calculated. The calculated HETP often exceeds 2m, leading to an excessively high packing height in the pumparound section, which is contrary to industry experience.

This presentation examines the reasons for the disproportionate packing HETP and consequently proposes a simple method to estimate the pumparound packing height based on an equilibrium stage simulation, thereby reconciling simulation and reality. A case study for the absorption of DMF from air into water will be presented to illustrate the design method.