Mass Transfer and Liquid Mixing in a Bubble Column with Vertical Tube Bundles

Bubble column reactors (BCRs) are multiphase contactors, used throughout the chemical process industry for reactions, such as, hydrogenation, oxidation and Fischer-Tropsch syntheses. The work in this report was done as part of a project at HZDR to investigate and further develop understanding of the impact of vertical tube bundle internals (for heat exchange), on the liquid mixing (LM) and mass transfer (MT) in bubble columns. Focus was placed on experimental and theoretical analysis of the liquid axial dispersion coefficient, Dz, and volumetric gas-liquid mass transfer coefficient, kla. A 10 cm diameter bubble column (DN100) was used with various typical tube bundle configurations (triangular and square pitch) and tube sizes (8 and 13 mm). As it is typical for industrial applications, one configuration was equipped with a U tube end and all internal configurations used constant coverage area of ~25 % (typical for the Fischer-Tropsch synthesis). All results were compared with the empty column to account for the influence of internals. A salt tracer pulse was used to introduce a change in liquid conductivity for LM experiments and a step change of oxygen concentration introduced to determine the MT coefficient. The axial dispersion and volumetric MT coefficient were estimated using the axial dispersion model (ADM) for LM and MT, respectively. Furthermore, radial effects were evaluated using the 2D ADM.
Key liquid mixing findings:

• Internals caused turbulence dampening and decreased dispersion in slug flow
• Mixing times increased with addition of internals
• The U tube end mitigated this in the bubble flow regime
• Radial effects were shown to factor in with internals at gas velocities > 10 cm s-1
• Validated 1D ADM for Dz estimation for the empty column from 2-20 cm s-1
• Strong recirculation was induced by the U tube end in the bubble flow regime

Key mass transfer findings:

• All internals configurations slightly increased mass transfer time
• kla data estimated using the ADM were consistent with literature
• CSTR model was found not to be applicable for the investigated columns

Square pitch 8 mm tubes with the U tube end was shown to be the optimum configuration.