Fluid Dynamic Parameters in Bubble Columns with Internals

The Knowledge of Gas Holdup, Liquid Recirculation and Turbulent Parameters is Important for Design and Performance Calculation of Bubble Column Reactors. Although Numerous Experimental Studies Have Been Reported on This Subject, Most Are Point Measurements Limited to Columns Without Internals Operated at Low Gas Velocities. in This Study, We Present the Results Obtained for the Gas Holdup Profiles, Time-Averaged Liquid Velocity Profiles, Turbulent Stresses and Eddy Diffusivities (Radial and Axial) Obtained in a 18″ (44 Cm) Diameter Column Without and with Internals Similar to Those Used in Industrial Scale Units (E.g., Heat Exchanger Tubes) using Both Air/water and Air/drakeoil 10 (Viscosity ~30 CP) at Gas Superficial Velocities of 2, 5 and 10 Cm/s. the Scale-Up Procedure Suggested by Degaleesan (1997) is Critically Examined in Light of These Results

The modelling of buoyancy driven flow in bubble columns

Using the analogy between lateral convection of heat and the two-phase flow in bubble columns, alternative turbulence modelling methods were analysed. The k-ε turbulence and Reynolds stress models were used to predict the buoyant motion of fluids where a density difference arises due to the introduction of heat or a discrete phase. A large height to width aspect ratio cavity was employed in the transport of heat and it was shown that the Reynolds stress model with the use of velocity profiles including the laminar flow solution resulted in turbulent vortices developing. The turbulence models were then applied to the simulation of gas-liquid flow for a 5:1 height to width aspect ratio bubble column. In the case of a gas superficial velocity of 0.02 m s-1 it was determined that employing the Reynolds stress model yielded the most realistic simulation results. © 2003 Elsevier B.V. All rights reserved

Recalcitrant bubbles

We demonstrate that thermocapillary forces may drive bubbles against liquid flow in ‘anomalous' mixtures. Unlike ‘ordinary' liquids, in which bubbles migrate towards higher temperatures, we have observed vapour bubbles migrating towards lower temperatures, therefore against the flow. This unusual behaviour may be explained by the temperature dependence of surface tension of these binary mixtures. Bubbles migrating towards their equilibrium position follow an exponential trend. They finally settle in a stationary position just ‘downstream' of the minimum in surface tension. The exponential trend for bubbles in ‘anomalous' mixtures and the linear trend in pure liquids can be explained by a simple model. For larger bubbles, oscillations were observed. These oscillations can be reasonably explained by including an inertial term in the equation of motion (neglected for smaller bubbles)

A Contextual Approach to Disobedience

Edmund Burke once noted that the rebelliousness of colonial America was largely a consequence of the size and prominence of the legal profession, under whose influence the people snuff the approach of tyranny in every tainted breeze. Today, however, most members of the legal profession take a much dimmer view of civil disobedience, although some do acknowledge its justification in special circumstances. Few who write on the subject recognize that in making judgments about the morality of disobedient acts the lawyer\u27s perspective is limited. Disputes over whether an illegal action is morally justified in a particular instance can be conceptually reduced to four basic areas of disagreement. 1. Is the disobedient act committed pursuant to a goal which is considered desirable and sufficiently important? One might condemn an illegal act to promote racial segregation on the ground that no efforts toward that end are morally defensible. Massive dislocation to improve food in the college cafeteria might be rejected because the goal, though desirable, is not very important. 2. What are the probable effects of disobedience? This is theoretically a factual inquiry, although the relevant facts may be unascertainable and highly complex. It encompasses both an inquiry into the likelihood that the actor\u27s goal will be achieved and an examination of other possible effects of the act. 3. Are the probable effects desirable? If particular consequences are predictable, one may think them good or bad. This evaluation will be particularly difficult if one value, such as equality, is promoted at the expense of another, for example security. And if among many consequences some are considered desirable and others undesirable, their respective importance must be weighed. 4. Is obedience to la* a moral claim on individuals that overrides moral claims in favor of disobedience? The thesis of this discussion, in large part, is that the answer to the last question depends on answers given to questions 2 and 3. Others, however, may believe it to be an independent inquiry. Indeed one possible position is that this is the only necessary inquiry; if one is always morally obligated to obey the law, goals and probable effects are irrelevant. Few, I think, would take this position upon serious reflection. Questions involving values are ones to which human reason can provide no final answers, and the factual inquiries involved in determining possible effects of disobedience are too complex for social science to give confident responses. Despite these difficulties, the second question is one with which the social scientist is best equipped to deal. The other three are most closely within the domain of the social and moral philosopher. Certainly no discipline has a monopoly on relevant truth or understanding in these areas. Although these inquiries are not central to his professional training, a lawyer is used to weighing values and making judgments about complex social facts. But that is hardly enough to justify the sublime confidence with which some members of the bar pontificate on the subject. Lawyers would do well to recognize that when they do discuss this topic, they are stepping beyond the narrow area of their particular professional competence. They should also be aware of the possibility that vocational commitment to the law is double-edged. At the same time it enriches comprehension of conflict management and the worth of orderly process, it may lead to a partial view of the relative importance of observance of legal norms as opposed to other social values. In this essay, I first suggest an unoriginal criterion for determining when disobedience of law is morally justified, one that is based upon the probable consequences of the disobedient act. This leads me to reject the conclusion advanced by some commentators that certain conditions, such as nonviolence and a willingness to be punished, are always essential if illegal behavior is to be justified. I then try to demonstrate why such factors are relevant, and often determinative, for justification. The application of the criteria advanced to specific disobedient acts does not produce simple answers to questions of moral justification. But simplicity of application, whatever may be its importance for legal norms, is not, in my view, a necessary hallmark of a principle of moral judgment

Continuous phase axial mixing in rotary disk contactors

This paper presents a review of important investigations on axial mixing in the continuous phase in rotary disk contactors (RDC) and analyses the various literature data in terms of the flow number (DR N/ue) and a geometry factor Gf. A new model equation has been developed to represent continuous phase axial mixing in the RDC taking into account the factors affecting the recirculation flow within the compartment and the main flow through the compartment in terms of disk-Reynolds number and modified tube-Reynolds number respectively. A satisfactory expression for Gf has been obtained on the basis of the present proposed model and analysis of data of available literature and those of the present work, covering laboratory and industrial size columns of 10 different investigators varying from 3.5 cm to 218 cm dia involving 19 different column geometries. The following forms of generalized correlation for continuous phase axial mixing in RDC have been suggested. (a) Ec/ucZc=0.5 + 0.028(DRN/uc) [Ds-0.5 Dr/Dr] (b) Ec/ucZc = 0.5 + 0.028 [(Re)R/(ReT)] (Ds/DR) - 0.5 (Dr/DR)

Continuous and dispersed phase coefficients for heat and mass transfer involving single-file drops

An attempt has been made in this paper at modelling analogous situations of heat or mass transfer involving single-file drops in liquid-liquid systems. Experimental data on continuous and dispersed phase coefficients have been correlated

Absorption of oxygen by aqueous sodium sulphite solutions

New data have been reported on the absorption of oxygen by aqueous sodium sulphite solutions with cobalt sulphate as catalyst at 30°C. It is observed that the absorption rate increases up to a critical sulphite concentration beyond which the absorption rate becomes constant and this critical concentration varies with the catalyst concentration. The order of reaction with respect to sulphite and oxygen in the constant absorption rate regime was found to be zero and two respectively. The absorption rate is affected by the total ionic strength (sulphite and sulphate) in addition to the effect of pH and catalyst concentraion. The reported data were also analysed to ascertain the effect of temperature and a relationship has been proposed for prediction of the second order reaction rate constant

Bubble swarm characteristics in bubble columns

New correlations have been proposed for estimation of gas phase holdup, characteristic velocity, interfacial area for mass transfer and mean bubble size of bubble swarms under dispersed and fluidized operation of bubble columns employing single - and multi-orifice distributors. The analysis of results include available literature data of other investigators