Determination of Effective Shear Rate in a Pilot-plant External Loop Airlift Reactor

Shear rate is an important design parameter in bio-reactors for its role in estimating cell-damage rate in shear-sensitive environment as well as in correlating hydrodynamics and mass transfer coefficients in non-Newtonian systems. One of the serious shortcomings in the analysis of non-Newtonian behaviour in airlift loop reactors is the lack of a reliable method for determining effective shear rate and viscosity appropriate to the airlift geometry.This work has been carried out in a pilot-plant external loop airlift reactor of 6.5 m in height and 0.225 m diameter of both riser and downcomer. Biological media were simulated using non-Newtonian solutions of xanthan gum and carboxymethyl cellulose. The resulting shear rates are compared to the available literature, and show an increase in shear rate with increasing superficial gas velocity. Also, it has been found that non-Newtonian solutions with similar rheological properties and different chemical structure have different shear rate trends at a given superficial gas velocity

Identification of hydrodynamics characteristics in bubble columns through analysis of acoustic sound measurements-Influence of the liquid phase properties

The effects of liquid properties on the hydrodynamics of bubble columns were investigated experimentally through analysis of acoustic sound measurements, using coalescence and non-coalescence mediums. The hydrodynamics parameters such as gas holdup, average bubble radius, gas bubbling rate, root mean square of the sound pressure and damping ratio of the bubble pulsation were investigated at the sparger and bulk regions. The acoustic study revealed that the addition of carboxymethyl cellulose (CMC) and xanthan gum (XG) in small percentage increased the overall gas holdup and reduced the average bubble radius. Moreover, the bubbling rate for these solutions is lower than that for water at low superficial gas velocities. These observations were more apparent in the CMC case. The injection of KCl and silicon polymer substances however, resulted in reduction of the gas holdup and enlargement of the bubble size. In addition, the bubbling rate for the KCl and silicon polymer solutions is found to be superior to that for water. In addition, it is found that acoustic measurements can be used to detect sparger activity for both air-water and non-Newtonian solutions systems. At low superficial gas velocity, the sparger acts as a nozzle, hence heterogeneous bubble size distribution was observed and detected. At high superficial gas velocity, the sparger becomes fully activated and consequently homogeneous size distribution was detected

Gas hold-up estimation in bubble columns using passive acoustic waveforms with neural networks

Passive acoustic waveforms produced experimentally from a bench-scale two-phase bubble column were recorded using a miniature hydrophone at three axial positions. The generated acoustic waveforms were processed and trained using artificial intelligence against global gas hold-up measurements. Two neural network architectures, the radial basis function (RBF) neural network and the recurrent Elman neural network, were employed. Both neural network techniques achieved accurate gas hold-up estimation, characterised by low mean square errors of 2.70 and 1.68% for the RBF and recurrent Elman networks respectively. The designed and trained neural networks were found to be a powerful tool for learning and replicating complex two-phase patterns. Passive acoustic waveforms were found to be a useful measuring technique for gas hold-up estimation in bubble columns under moderate operating conditions. © 2006 Society of Chemical IndustryInternational Journa

Prediction of regime transitions in bubble columns using acoustic and differential pressure signals

This paper addresses the identification of the transition points of the flow regimes in bubble columns. For this purpose, statistical and spectral analysis of the acoustic sound signals and differential pressure signals were employed over several values of the gas velocity. Both analysis tools were able to extract useful qualitative and quantitative information about the flow patterns. In addition, applying the two methods to both types of signals presented comparable results about the quantitative values of the critical velocities. These values agree well with those reported in the literature. Moreover, the implementation of the auto-correlation function on the differential pressure signal was found helpful in determining period of the macro-structure fluctuation

Effect of antifoam agents on bubble characteristics in bubble columns based on acoustic sound measurements

In this paper, the effect of antifoam agents on bubble characteristics in bubble columns is studied. Specifically, the bubble characteristics of air in tap water are compared to those of air in 5% and 10% antifoam solutions. Bubble characteristics such as gas holdup, bubble diameter, bubble-size distribution, and damping ratio were investigated at various superficial gas velocities. These properties were deduced from the acoustic sound measurement. The study revealed that the addition of antifoam chemicals reduces the overall gas holdup and increases the average bubble diameter. The bubble-size distribution in tap water is found to be homogeneous while in antifoam solutions to be heterogeneous. It is also found that at low gas velocities the damping ratio for antifoam solutions is higher than that for tap water, while at high gas velocities the damping ratio is not affected. The results affirm that acoustic probes are excellent measuring tools over classical tools at moderate gas velocities

Preparation of the chitosan containing nanofibers by electrospinning chitosan-gelatin complexes

Electrospinning is an interesting technique, which provides a facile and an effective mean in producing nonwoven fibrous materials; however, for producing nanofibers, investigation of the electrospinning conditions is very important. In this study, chitosan, gelatin, and their polyelectrolyte complexes (PECs) were electrospun to prepare nonwoven nanofibrous mats. The concentrations of chitosan and gelatin solutions and electric field (kV/cm) were optimized. The solutions were then blended in different ratios (0-100%) to get electrospun nanofibrous mats. Solution concentration and electric field showed pronounced effect on the electrospinnability and fiber diameter of these systems. Mostly large beads coexisted with the fibers were observed for chitosan at 1 wt% solution concentration, which then showed good electrospinnability at 2 wt% (nanofiber diameter was 145 and 122 nm at 15 and 20 kV/10 cm, respectively), whereas gelatin showed no electrospinnability below 15 wt% solution concentration and a homogenous fibers network at 15 wt% (149 nm at 20 kV/10 cm). The morphology and diameter of chitosan-gelatin PEC nanofibers varied with the chitosan/gelatin ratio. The crystallinity of chitosan was also observed to reduce with electrospinning and addition of gelatin

Crystallinity and morphological evolution of hydrothermally synthesized potassium manganese oxide nanowires

Potassium manganese oxide (KMn8O16) nanowires were synthesized using a customized hydrothermal method and characterized using scanning-electron microscopy, X-ray diffractometry and thermogravimetric analysis to determine the effects of reaction temperatures and molar ratio of reactants on the crystallinity and morphology of the synthesized nanowires. It was established that increasing the stoichiometric portion of potassium precursors increased the average nanowire diameter though such effect was comparatively less prominent in terms of reaction temperature. Deficient supply of potassium inhibited nanowires growth in which only KMn8O16 (cryptomelane) growth orientations of (211), (301) and (600) were observed along with traces of MnO2, resulting in a wool-like nanowires suspension