(617hp) Towards Industry-Scale Measurements of Reactor Gas-Liquid Mass-Transfer Phenomena

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 the case for research to intensify mass transfer in bioreactors that upgrade natural gas. The mass transfer rate can be enhanced by increasing interfacial area a, by increasing reactor pressure, and/ or by intensifying the liquid-side mass transfer coefficient k_L. In the first part of this work the goal is to achieve a volumetric mass transfer coefficient (k_La) above 2,000 1/hr with better energy efficiency than current industrial options. To increase mass transfer rate to k_La > 2000 1/hr, microbubbles (< 200 micron) were generated in a 100L bubble column reactor while maintaining low power consumption. The volumetric surface area is inversely proportional to the bubble diameter. Therefore, microbubbles offer larger gasâ??liquid interfacial areas and longer residence times compared to conventional larger bubbles. Gas-liquid-transfer coefficient (k_La) and bubble dispersion hydrodynamics were measured under a spectrum superficial liquid and gas velocities. Non-chemical experimental methods for measuring reactor volume-averaged mass transfer coefficient are limited to small values of k_La. In order to measure k_La accurately in the range >2000 1/hr, a chemical reactant is necessary to buffer the rise of dissolved gas. Further, control of the reaction rate is necessary in order to ensure the Hatta number and other governing parameters of the process are appropriate (Gaddis 1999, Sharma 1969), which will be explained in this talk. Here we present new data and methods of the [Na₂SO₃, CoSO₄, O₂] system for k_La measurement. Sulfite measurements of k_La are not uncommon in industry and literature, but are hindered by sensitivity to impurity of the catalyst, which necessitates costly and prohibits experiments in metal reactors. In the second part of this work the goal is to improve the sulfite measurement of k_La via a novel electrowinning method that allows in-situ cleaning (CIB) of the test solution in the reactor. This new method allows for use of less expensive reagents and possibly for use of stainless steel reactor materials, both of which greatly facilitate the [Na₂SO₃, CoSO₄, O₂] system in larger industrial reactors. We present data from successful tests of this electrowinning system, and new kinetics data to constrain the reaction order with respect to the gas m, order with respect to the liquid n, and order with respect to cobalt l. It was observed that electrowinning reduces background impurities of the chemical system, causing a decrease in absorption rate by a factor of O(10) and allowing use of less expensive sulfite. The electrowining method was scaled up from bench-top (0.5L) to the larger reactor (100L). The electrowinner design and results are presented.

References:

Gaddis, E.S. 1999 Mass transfer in gasâ??liquid contactors. Chemical Engineering and Processing. 38, 503â??510.

Sharma, M.M, Dankwerts, P.V. 1969 Chemical methods of measuring interfacial area and mass transfer coefficients in two-fluid systems. British Chemical Engineering. 15, 4, 522-528.