(30e) Effect of Reactor Height on Intensification of Mass Transfer Coefficients (kLa, kL) in a Turbulent Inverse Bubble Column

In many gasâ??liquid contactors or reactors, mass transfer from the gas phase to the liquid phase is a major player in the reactorâ??s overall production and/or economics. This talk presents chemical and hydrodynamic data from research to intensify gas-liquid mass transfer in an inverse bubble column bioreactor meant to upgrade natural gas. We measured k_La by dynamic oxygen absorption and turbulence properties by high-speed camera particle imaging velocimetry (PIV) for four reactor heights. Physical understanding is sought by linear addition of previous theories of k_L under turbulent conditions (Calderbank & Moo-Young 1961) or bubble terminal rise conditions (Clift 1978) to determine k_La and turbulence intensity at various locations in the reactor. We also add entrance effects according to Higbieâ??s â??penetration theoryâ? for interfacial transfer, wherein the freshly created gas-liquid interface leads to higher absorption rate than the classic Calderbank or Froessling type of equations. However, this entrance region transfer rate does not persist long enough (or cover a large enough volume fraction of the reactor) to be dominant over the mass transfer that is forced by other phenomena such as turbulence or bubble-rise-velocity. We observed k_La values to decrease as the height of the reactor increased from 31 to 124 cm. A model was constructed in which k_L is a sum of effects from bubble terminal rise velocity, turbulence and entrance effects. As a typical bubble moves from jet region (high turbulence) to the bottom of the reactor (low turbulence), energy dissipation decreases, which leads to a decrease in k_La value. The modeled data match experimental data well. Based upon the results we hypothesize that the best method for intensification of k_La is by increase of a because the effect of turbulence on k_La is muted by a ¼ exponent. Turbulence and bubble rise velocity is found to mediate k_L in most of the reactor.

References:

Calderbank, P.H. & Moo-Young, M.B. 1961 The continuous phase heat and mass transfer properties of dispersion. Chem. Engng Sci. 16, 39-54.

Clift, A., Grace, J.R. & Weber, M.E. 1978 Bubbles, Drops, and Particles. Academic Press, Inc. New York.

Higbie, R. 1935 The rate of absorption of a pure gas into a still liquid during short periods of exposure. Transactions of the American Institute of Chemical Engineers. 31, 365.