Three-Phase Fluidized Bed—a device in which the gas phase moves in the form of bubbles relative to the liquid phase, and eventually reactive solid is fluidized in the liquid phase—has varied commercial applications such as in heavy oil, resid hydroprocessing, synthetic crude processing, coal liquefaction in the presence of catalyst, biological waste water treatment and fermentation.Its design and operation depends on the effect of operating variables such as gas and liquid velocities, particle properties and liquid properties on the heat and mass transfer characteristics. Normally, the heat transfer coefficient of a three-phase fluidized bed is an increasing function of the respective gas and liquid velocities, the size and density of particles, column diameter and the thermal conductivity and heat capacity of the liquid phase.

Although many experiments were carried out and correlations proposed on wall-to-bed mass transfer in three-phase fluidized beds, there are hardly any studies assessing the effect of viscosity on wall-to-bed mass transfer coefficient. It is to fill this gap the authors, G N Ravi Shankar, G M J Raju, M S N Murty and K V Ramesh, of the first paper, “Effect of Viscosity on Wall-to-Bed Mass Transfer Coefficient in a Three-Phase Fluidized Bed”, have carried out investigations choosing an electrolyte—an equimolar solution of potassium ferricyanide and potassium ferrocyanide of 0.01 N with 0.4 N NaHCO3/Na2CO3—as liquid phase, glass spheres of different diameters as solid phase and inert nitrogen as gas phase. In order to alter the viscosity, carboxy methyl cellulose sodium salt in 0.1, 0.2 and 0.4 wt% was added to the electrolyte. The experiment was conducted at a constant temperature of 25 oC. The investigations revealed that the mass transfer coefficient was relatively independent of the liquid velocity within the range covered under the experiment; the mass transfer coefficient increased with increase in gas velocity, of course approaching a constant value; and the mass transfer coefficient increased with increase in particle size and decreased with increasing carboxy methyl cellulose concentration.

Moving on to computational fluid dynamics, we have K Navas Mohamed and P Sivashanmugam, the authors of the next paper, “CFD Simulation of Subcooled Vertical Nucleate Flow Boiling of Water”, who carried out CFD studies on subcooled nucleate flow boiling of water at elevated pressure to study the volume fractions, velocities, and temperature distributions using GAMBIT 2.3.16 for the geometry creation and meshing, while solver and post-processing were done using FLUENT 6.3.26. Their study revealed that the subcooled water enters the pipe followed by boiling due to the uniform heating from the pipe wall. Since the wall was heated, bubbles formed adjacent to the wall moving later towards the center. The liquid velocity was found to increase along the pipe as the void fraction increased. Maximum liquid velocity was noticed near the center of the wall.

The simulated void fraction profile was in agreement with those in the literature.Shifting from CFD studies to semifluidization beds—a combination of fluidized bed at the bottom and packed bed at the top, in series within a single contacting vessel that is supposed to overcome the inherent disadvantages of a packed bed as well as that of a fluidized bed—we have A B Soni, H Kumar and Pooja V Shrivastava, the authors of the next paper, “Studies on Momentum Transfer Aspects of Semifluidized Beds in Annular Sections”, presenting the results of their study on momentum transfer aspects of semifluidized beds in annular sections.

By collecting data on variables such as liquid mass velocity, particle size and particle density, the authors have developed correlations adapting dimensional analysis approach for the prediction of pressure drop and of minimum and maximum semifluidized velocity. The calculated data was found to be in agreement with the data obtained from the experiments.

Moving on to plasma gasification—an upcoming environment friendly waste treatment technology that is known to be of extremely low carbon emission—we have Mehali J Mehta, Robin A Christian, Narendra J Mistry and Mausumi Mukhopadhyay, the authors of the next paper, “Plasma Gasification: A Waste Treatment Technology”, reviewing the efficacy of plasma gasification for treating municipality waste, treatment of hazardous waste, the by-products thereof, advantages and disadvantages of its adoption.

Lastly, we have Ajit P Rahod, Kailas L Wasewar and Shriram S Sonawane, the authors of the last paper, “Pervaporation Reactor: Principles and Applications”, reviewing the principles and applications of pervaporation reactor, an important membrane separation process, for effective separation of various industrial products that has a good potential for enhancing conversion in esterification reaction coupled with more energy efficiency.

-- GRK Murty
Consulting Editor