Axial mixing of liquid in gas-liquid flow through packed beds

Parametric Mathematical Modelling of Cristal Violet Dye Electrochemical Oxidation Using a Flow Electrochemical Reactor with BDD and DSA Anodes in Sulfate Media

An important issue in electrochemical oxidations of pollutant compounds, like organic dyes, is identifying a suitable correlation between operational conditions and electrochemical process performance. In such sense, this work deals with the parametric modelling of direct electrochemical incineration of crystal violet (CV) dye in a FM01-LC flow electrochemical reactor with a plastic spacer configuration using boron doped diamond (BDD) and dimensionally stable (IrO2 and IrO2-SnO2-Sb2O5) anode plates. Mathematical model takes into account the fluid dynamics effects by the use of FM01-LC reactor considering mass transport rate of organic compound (R) from bulk solution to electrode surface, characterized by a dispersion coefficient and Pe number. The effect of strong oxidants produced in the electrode surface can be neglected since the characteristic time constant reaction of pollutants with such oxidants is lower than those describing the diffusion of organic compound to the electrode surface. Model parameters were estimated throughout a fitting method of the experimental data. The model proposed here predicted a 99.7 removal percentage of CV with boron doped diamond and IrO2-SnO2-Sb2O5 anodes obtained experimentally, meanwhile a 79 % removal with the IrO2 anode was reached at Re = 2204 during an electrolysis time of 7200 s for both cases. In the case of IrO2 anodes, complex interactions between hydroxyl-radical and electrode surface provokes an intermediate kinetic process, with an effectiveness factor of 0.59. When BDD and IrO2-SnO2-Sb2O5 anodes were used, the removal process mediated by hydroxyl-radicals absorbed in electrode surface was fully limited by mass transport.

Investigation of flow dynamics of liquid phase in a pilot-scale trickle bed reactor using radiotracer technique

A radiotracer investigation was carried out to measure residence time distribution (RTD) of liquid phase in a trickle bed reactor (TBR). The main objectives of the investigation were to investigate radial and axial mixing of the liquid phase, and evaluate performance of the liquid distributor/redistributor at different operating conditions. Mean residence times (MRTs), holdups (H) and fraction of flow flowing along different quadrants were estimated. The analysis of the measured RTD curves indicated radial non-uniform distribution of liquid phase across the beds. The overall RTD of the liquid phase, measured at the exit of the reactor was simulated using a multi-parameter axial dispersion with exchange model (ADEM), and model parameters were obtained. The results of model simulations indicated that the TBR behaved as a plug flow reactor at most of the operating conditions used in the investigation. The results of the investigation helped to improve the existing design as well as to design a full-scale industrial TBR for petroleum refining applications.

Axial dispersion in packed bed reactors involving viscoinelastic and viscoelastic non-Newtonian fluids

Axial dispersion is an important parameter in the performance of packed bed reactors. A lot of fluids exhibit non-Newtonian behaviour but the effect of rheological parameters on axial dispersion is not available in literature. The effect of rheology on axial dispersion has been analysed for viscoinelastic and viscoelastic non-Newtonian fluids. Aqueous solutions of carboxymethyl cellulose and polyacrylamide have been chosen to represent viscoinelastic and viscoelastic liquid-phases. Axial dispersion has been measured in terms of BoL number. The single parameter axial dispersion model has been applied to analyse RTD response curve. The BoL numbers were observed to increase with increase in liquid flow rate and consistency index 'K' for viscoinelastic as well as viscoelastic fluids. Bodenstein correlation for Newtonian fluids proposed has been modified to account for the effect of fluid rheology. Further, Weissenberg number is introduced to quantify the effect of viscoelasticity.

The influence of complex biological feedstock on the fluidization and bed stability in expanded bed adsorption

The stability of expanded bed adsorption systems (EBA) was studied in biomass containing culture broth by residence time distribution (RTD) experiments, using pulse inputs of fluorescent molecules as tracers. Different commercial adsorbents (Streamline DEAE, SP, Phenyl, Chelating, and AC) were tested at various biomass concentrations (2.5-12 %, wet weight) of whole (Saccharomyces cerevisiae) yeast, yeast cell homogenate, and Escherichia coli homogenate. Analyzing the RTD according to the PDE model (PDE: axially dispersed plug-flow exchanging mass with stagnant zones) allowed the calculation of three parameters: the number of transfer units for mass exchange between mobile and stagnant fraction (N), the Peclet number for overall axial dispersion (P), and the mobile fraction of the liquid in axially dispersed plug flow (varphi). When fluidization was performed in particle-free buffer the normalized response signal (after perfect input pulse) was symmetric (N:0; P: 50-100; varphi: 1), thus, demonstrating the formation of a homogeneous fluidized (expanded) bed. Upon application of suspended biomass the RTD was skewed, depending on the adsorbent used and the type and level of biomass present in the sample. This situation leads to three different characteristic pictures: the well-fluidized system (N: >/= 7-10; P: >/= 40; varphi: 0.80-0.90), the system exhibiting bottom channeling (N: < 1-2; P: >/= 40; varphi: 0.5-0.7) and, the system where extensive agglomeration develops (N: 4-7; P: 20-40; varphi: < 0.5). These results demonstrate that changes in the hydrodynamics of EBA already take place in the presence of moderate concentrations of biomass. Furthermore, those changes can be quantitatively described mainly in terms of the fraction of stagnant zones in the system, which are formed due to the interaction of biomass and adsorbent. The technique described here can be used to evaluate a certain combination of adsorbent and biomass with regard to its suitability for expanded bed adsorption from whole broth. Copyright 1999 John Wiley & Sons, Inc. Biotechol Bioeng 64: 484-496, 1999.