Article pubs.acs.org/IECR
Effects of Orifice Angle and Surface Roughness on the Bubbling-toJetting Regime Transition in a Bubble Column Carlos Irrgang,† Olaf Hinrichsen,† and Raymond Lau*,‡ †
Department of Chemical Engineering, Technische Universität München, Lichtenbergstrasse 4, 85747 Garching, Germany School of Chemical and Biomedical Engineering, Nanyang Technological University, 62 Nanyang Drive, Singapore 637459
‡
ABSTRACT: Submerged gas injection into liquids is a widely applied processing technique. The main objective of the current study was to determine the effects of orifice angle and orifice surface roughness on the bubbling-to-jetting regime transition in a bubble column. The bubbling behaviors at single-orifice distributors were investigated over a wide range of orifice gas velocities. The pressure fluctuations in the plenum and high-speed image sequence of the bubble emerging process at the orifice were recorded in experiments. Both orifice angle and orifice surface roughness were found to have significant effects on the regimetransition velocity between the bubbling and jetting regimes. Models were also developed by incorporating those available in the literature with the experimental results obtained in the current study.
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INTRODUCTION Operations involving mass or heat transfer across an interface are very common in chemical industry. To obtain a high transfer rate, a large interfacial area per unit volume is preferred. Three methods are commonly used to satisfy this requirement: the film method, the rupture of bulk fluid, and the dispersion of gas through submerged orifices. Of these, gas dispersion through submerged orifices is the most efficient and most commonly used. The design of the corresponding equipment is simple and leads to reasonably large interfacial areas. The principle applies mostly in processing equipment such as distillation columns, bubble columns, absorption towers, flotation cells, airlift vessels, and aerated stirred tanks. Thus, the formation of bubbles and gas jets, which is the first step in gas dispersion, is a very important aspect in the study of dispersion process. In the design and operation of gas−liquid contacting equipment, it is essential to clarify the factors affecting the formation of gas jets and to understand the underlying mechanisms that cause the transition between the bubbling and jetting regimes. Although practical applications usually involve the simultaneous participation of many orifices, most experimental and theoretical studies of bubble formation have been concerned with single orifices.1−5 Studies involving multiple orifices are complicated, and drawing definite conclusions from such studies is difficult. Hence, studies of bubble formation at single orifices are performed to exclude the mutual influence of bubbles formed in neighboring orifices. Although the effects of adjacent orifices are neglected, the study of bubble/jet formation at a single orifice yields statistical information concerning the affecting factors and also provides insight into the dynamics of the process under multiple-orifice conditions. As the number of orifices increases in multipleorifice conditions, the plenum volume per orifice decreases. The liquid weeping rate is reported to increase with a reduction in plenum volume.6 Thus, it is anticipated that the regimetransition velocity will increase with increasing number of orifices. One other important application of the knowledge of © 2012 American Chemical Society
the regime-transition velocity is the interruption of liquid weeping, an undesired phenomenon, with the least energy. Many industrial bubble-column processes involve flow of both liquid and gas through the orifices. Weeping can occur if the liquid is static or injected above the gas distributor unless the gas flow rate is maintained above the regime-transition velocity. Three main bubbling regimes, namely, static, dynamic, and jetting, are observed in order of increasing gas flow rate.2 The static regime is also called the constant-volume regime and occurs when the Reynolds number (Re = UoρgDo/μg, where Uo is the interstitial velocity of the fluid at the orifice, Do is the orifice diameter, ρg is the gas density, and μg is the gas viscosity) is less than 100.2,7 The static regime occurs under conditions where only bubble buoyancy and surface tension play significant roles and there is equality between these two forces throughout bubble formation. The gas flow rate is normally very low (
