Mass transfer characteristic and design of bubble column reactor in homogeneous and heterogeneous flow regimes

Abstract

A bubble column reactors have gained considerable attention due to its excellent mass transfer characteristics, simple operation and large gas-liquid interfacial area. The understanding of flow hydraulics is extremely important in the bubble column operation. However, comprehension of the bubble column hydrodynamics is yet to be in clear for industrial applications. Therefore, in order to provide a framework for more detailed consideration of hydrodynamics in the bubble column reactor. We focused on following topics.

First, mass transfer behavior on the bubble column was investigated. Either monoethylene glycol (MEG) solution or n-hexane was adopted as a liquid phase and argon was used as a gas phase. Experiment measuring mass transfer parameter revealed that the gas holdup and the mass transfer coefficient became lowered by the static liquid height increase due to the increase of bubble size. A sieve tray was introduced to offset the loss of mass transfer caused by static liquid height increase. Two sieve trays having different hole sizes were employed. Standard deviation of pressure and the bubble diameter were observed to understand the effect of the sieve tray on the mass transfer. Employing smaller size hole sieve (1 mm) resulted in reduced mass transfer due to the formation of gas cap, which appear below the sieve. Mass transfer was enhanced due to the reduction of bubble diameters without any gas cap formation by installing larger hole sieve (3 mm),. However, the effect of the sieve tray was weakened when the flow regime changes from homogeneous to heterogeneous.

Secondly, the transition of flow regimes was investigated with various gas-liquid systems in a bubble column. Analyzing the transition regime in the bubble column is important issue because considerable change in bubble characteristic and mass transfer behavior is accompanied. Various parameters of gas holdup, volumetric mass transfer coefficient, drift flux and pressure standard deviations in an acrylic column of 1.8 m in height and 0.2 m in diameter were investigated to precisely determine superficial gas velocity at the transition regime. Under the range of 2.5 to 5 in the ratio of static liquid height to diameter (H/D), the superficial gas velocity of the transition regime generally becomes higher with the lower liquid height. It was observed that obtained transition regime velocities determined by different parameter had similar value. Two correlations to decide gas holdup of homogeneous and heterogeneous flow regime were derived from the investigated gas holdup experimental data in this work. The correlation to determine the transition regime velocity was also suggested using the gas holdup correlation for homogeneous and heterogeneous flow regime. Finally, bubble column with an external loop another type of bubble column, was investigated. The gas holdup of the BCR and BCR-EL was compared while maintaining same volume of the liquid phase. The riser and downcomer gas holdup in bubble column with external loop was less than gas holdup in original bubble column. However, result of gas desorption concentration measurement under same gas phase input revealed that reactor performance could be better even in high superficial gas velocity (0.10 m/s) operating with bubble column with an external loop. The observation of rise and descend velocity of bubble showed that ratio of bubble pass the downcomer continuously increase with increasing superficial gas velocity. For a sophisticated comparison, reactor modeling was carried out with Fisher-Tropsch synthesis reaction. The simulation result prevailed that the syngas conversion was high in BCR at homogeneous regime. In heterogeneous regime, BCR-EL has higher performance than BCR since high gas recycle to downcomer results in high syngas conversion ate the reactor bottom.