Letters to the editor

Rice, Richard G.; Leighton, David T.; McCready, Mark J.; Tavare, N. S.; White, E. T.; Randolph, A. D.

doi:10.1002/aic.690350427

Cited by 3 publications

(2 citation statements)

References 6 publications

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“…In a recent published letter, Rice (1989) suggested the entrance and smooth zones could be treated as a series of resistances, so that (7) The mixing coefficients are Do (overall), D,, (entrance zone) and D, (Fickian zone) respectively. Also, L, is the total length, L, is the entrance length, and the length for smooth Fickian movement is L = L, -L,.…”

Section: Scaling Lawmentioning

confidence: 99%

Correlation of nonverticality and entrance effects in bubble columns

1990

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The prediction of mixing in bubble columns, especially that of axial dispersion coefficient, has received considerable attention in recent years, and it is clear that wide disagreement exists. A recent compilation of many results (Rice and Littlefield, 1987) showed order of magnitude differences among several research groups.Generally, mixing in unstirred vessels has been correlated with respect to two key parameters: column diameter and superficial gas velocity. However, two recent papers (Tinge and Drinkenburg, 1986;Rice and Littlefield, 1987) have shown that slight departure from true verticality ( t 2 degrees) had a much stronger effect than diameter and velocity. However, the former used very small diameter columns, while the latter presented sparse results. Predating these mixing studies, Valdes-Kreig et al. (1975) observed that vertical misalignment can strongly affect surfactant removal in a foam fractionation column.In this study, we present new visual VCR experimental evidence, using the acid-base technique described by Rice and Littlefield (1987). This technique allowed the following aspects to be studied:Entrance length for fully developed bubbly flow. Measurement of dispersion coefficient within the bubbly flow region, thereby mitigating inflation of values caused by the entrance region.Testing the "summation of resistance" hypothesis for twozone bubble columns.Measurement of enhanced mixing times and effective dispersion coefficient for controlled column tilt. Measurement TheoryThe basis for the batch acid-base technique is to set up two separated regions driven by the concentration gradient of AIChE Journal September 1990Fickian-like character. The measurement of the movement of the boundary can then be related to the effective dispersion coefficient. Baird et al. (1976) were perhaps the first to use the acid-base technique for dispersion measurement, although their solution of the Fickian equations differs slightly from that of Rice and Littlefield (1987) who gave the following result:where the pair [zn, t,] portrays the position-time movement of the LON (line of neutralization). For long times, because D, t,/ Lz > 0.3 the series terms can be ignored, so that A "one-shot'' experiment can be performed without using a VCR by measuring the time to neutralize the total column contents, denoted as t $ so when z, = L, then we have

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Section: Scaling Lawmentioning

confidence: 99%

Correlation of nonverticality and entrance effects in bubble columns

1990

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“…Both VS and LS are key characteristics of the micro-scale eddies. It is noteworthy that Rice [16] postulated that one of the substantial prerequisites for the local isotropic turbulence is that the turbulent parameter LS should be equivalent to the mean bubble diameter. According to the author, this is especially valid for truly vertical BCs with high-quality GDs.…”

Section: Theory Of Local Isotropic Turbulencementioning

confidence: 99%

Identification of Local Isotropic Turbulence Conditions in Various Bubble Columns Based on Several Reliable Parameters

Nedeltchev

2023

Fluids

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Bubble columns (BCs) are widely used in the chemical industry. In many industrial applications, these important gas-liquid contactors operate in a churn-turbulent flow regime. In principle, it is essential to determine the operating conditions in every BC reactor, in which local isotropic turbulence is established. In this work, it was demonstrated that several different parameters (Kolmogorov entropy, correlation dimension and novel hybrid index) follow a monotonic decreasing trend. This finding could be explained by the constantly increasing coalesced bubble size, which brings more order into the gas-liquid system and thus any entropic or chaotic parameter should decrease with the increase in the superficial gas velocity Ug. The profiles of the new parameters in various gas-liquid systems were studied. They were extracted from different pressure signals (gauge or absolute). In this research, BCs of different diameter and equipped with different gas distributors were used. It was demonstrated that the studied parameters could be successfully correlated with the length scale of the micro eddies and thus the Ug range of applicability of the local isotropic turbulence theory under various operating conditions was indirectly determined. The overall gas holdup profiles were analyzed and, based on the exponent of the Ug value, it was found that in the aqueous solutions of alcohols studied, the conditions in the bubble bed (BB) are homogeneous, whereas in the air-tap water system aerated in different BCs, the conditions in the BB are heterogeneous. This result implies that the local isotropic turbulence conditions predominate mainly around the corresponding measurement positions.

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