Porous Gas Distributor Research Articles

ECVT imaging of 3D spiral bubble plume structures in gas‐liquid bubble columns

AbstractElectrical Capacitance Volume Tomography (ECVT) is a 3D, real‐time imaging technique that is recently developed to image the multiphase flow behaviour in columns of regular or irregular geometries. In this study, the ECVT measurements are conducted to obtain phase holdups and flow structures in bubble columns of a straight‐cylinder shape and a tapered‐cylinder shape with porous and orifice gas distributors. The phase holdups obtained by the ECVT in a straight‐cylinder bubble column with porous gas distributor are verified by other measurement techniques and correlation equations reported in the literature. The flow regimes of the bubble columns are also characterized by the ECVT. Particularly, a dynamic 3D flow structure, which is uniquely represented by the spiral motion of the bubble plumes occurring within the heterogeneous regime of the gas‐liquid flow in a straight‐cylinder shape bubble column with perforated distributor, is captured for the first time instantaneously over the entire flow field with the ECVT. Other flow behaviour such as the converging flow pattern of the gas bubbles in a tapered‐cylinder shape bubble column is also revealed by the ECVT.

Water profile determination in a running PEMFC by small-angle neutron scattering

The water management in an operating polymer electrolyte fuel cell was studied by small-angle neutron scattering (SANS). The experiments were conducted at 80 °C under pressure (1.5 bar) using non-humidified gases and porous gas distributors. The parameters under study were the gas distributor porosity, the membrane thickness, the nature of the electrodes and the gas flow. SANS is shown to be a powerful technique to determine in situ the amount of water in the cell, to differentiate the water located within and outside of the membrane and its distribution across the membrane thickness. The data analysis leads to the determination of water concentration profiles across the membrane that could be used to validate mass transfer models. It is shown that a significant current can be extracted with a dry membrane, porous gas distributors and dry gases and that the membrane does not significantly swell whatever the electric load.

Polymer Electrolyte Fuel Cells With Porous Materials as Fluid Distributors and Comparisons With Traditional Channeled Systems

In this study, a novel concept is investigated, according to which the traditional ribbed flow delivery systems are replaced with permeable porous fluid distributors, which circumvent a number of known performance hindering drawbacks. A thorough single-phase model, including the conservation of mass, momentum, energy, species, and electric current, using Butler-Volmer kinetics, is numerically solved in three dimensions, to investigate the impact of different flow configurations on the performance of hydrogen fuel cells. It is found that cells with porous gas distributors generate substantially higher current densities and therefore are more advantageous with respect to mass transfer. Another advantage of porous flow distributors is the potential for higher power densities and reduced stack weight.

Metabolism of hybridoma cells and antibody secretion at high cell densities in dialysis tubing

The experimental setup, consisting of a bundle of dialysis tubing 2.5 mm in diameter [10–15 kD cutoff, mean pore size 25 Å, 20 μm (dry) and 40 μm (wet) wall thickness] inserted into a 1-l glass bioreactor supplied with oxygen and pH electrodes, a porous gas distributor, a sampling tube, and a holder for the eight pieces of dialysis tubing, was developed to investigate the properties and the microenvironment of hybridoma cells enclosed in the tubing during their batch cultivation. The concentrations of low-molecular-weight medium components were the same inside and outside the tubing, and it was possible to control the microenvironment of the cells in the tubing easily. The cell damage caused by mechanical stress was less in the dialysis tubing than in stirred spinner flasks. The influence of the initial cell density in the range from 4 × 10 5 to 1 × 10 8 cells ml −1 and the cultivation time were evaluated according to the total and viable cell concentrations and the cell/cell fragment size distributions. Furthermore, the cell membrane properties, glucose consumption rate, lactate, ammonia and lipid storage material, and the monoclonal antibody production rates as well as intracellular enzyme activities in the culture medium were measured and compared to those in reference cultures in spinner flasks with the same inoculum at low initial cell densities. In dialysis tubing in a concentration range of 5 × 10 6 to 10 8 cells ml −1, the total and viable concentrations of cells remained the same during cultivation. At high cell concentrations, cell size decreased, membrane folding increased, the fraction of dead cells and cell fragments increased, and the metabolic activity of the cells and the monoclonal antibody productivity were higher than with low cell concentrations. Since the dialysis membrane is nonpermeable for proteins, high monoclonal antibody concentrations were attained.

Porous gas distributors in bubble columns. Effect of liquid presence on distributor pressure drop. Effect of start‐up procedure on distributor performance

AbstractMany bubble columns use porous gas distributors to obtain small bubbles and large mass transfer rates. The distributor pressure drop contributes greatly to the energy consumption of bubble columns. This study found that the presence of liquid increased this pressure drop by 200 to 900% at practical gas flowrates. A model was developed to predict this increase. It assumes that gas flows through a fraction only of the pores so as to minimize the distributor pressure drop.The mass transfer performance of a porous distributor is affected by its startup procedure. The best performance was obtained by starting the gas flow before liquid was introduced into the column. A model was developed to explain these results.

Absorption and reaction of isobutene in sulfuric acid—III: Considerations on the scale up of bubble columns

In spite of their simple construction the scale up of bubble columns of industrial size demands application of models which account for dispersion effects and variations of pressure and gas flow rate. However, using such models and parameter values obtained from other studies it was not possible to describe successfully measured conversions of the absorption of isobutene in a 7 m bubble column though the interfacial area was determined separately. The measurements were carried out under such conditions at which the absorption takes place in the slow reaction regime of mass transfer. A sufficient agreement between experimental and predicted conversions could be obtained merely if a lower value of k L was used. A more detailed analysis of bubble size distributions indicated that the decrease of k L may be apparently only since the interfacial areas determined photographically must not necessarily be the area which is effective to mass transfer. k La -values in larger bubble columns with gas spargers which are common in industry are considerably lower than k La -data found in smaller columns with porous gas distributors.