A quantitative review of the transition salt concentration for inhibiting bubble coalescence

Abstract

Some salts have been proven to inhibit bubble coalescence above a certain concentration called the transition concentration. The transition concentration of salts has been investigated and determined by using different techniques. Different mechanisms have also been proposed to explain the stabilising effect of salts on bubble coalescence. However, as yet there is no consensus on a mechanism which can explain the stabilizing effect of all inhibiting salts. This paper critically reviews the experimental techniques and mechanisms for the coalescence of bubbles in saline solutions. The transition concentrations of NaCl, as the most popularly used salt, determined by using different techniques such as bubble swarm, bubble pairs, thin liquid film micro-interferometry were analysed and compared. For a consistent comparison, the concept of TC95 was defined as a salt concentration at which the -percentage coalescence‖ of bubbles reduces by 95% relative to the highest (100% in pure water) and lowest (in high-salt concentration) levels. The results show a linear relationship between the TC95 of NaCl and the reciprocal of the square root of the bubble radius. This relationship holds despite different experimental techniques, salt purities and bubble approach speeds, and highlights the importance of the bubble size in bubble coalescence. The available theoretical models for inhibiting effect of salts have also been reviewed. The failure of these models in predicting the salt transition concentration commands further theoretical development for a better understanding of bubble coalescence in salt solutions.

The dual effect of viscosity on bubble column hydrodynamics

“…This concept is further elaborated in Section 3.4. Generally, these results suggest that the stabilization of the homogeneous flow regime and, thus, the changes in the gas holdup are caused by the modifications to the BSDs and are induced by the coalescence suppression/enhancement mechanisms (connected to the bubble interface properties, (Firouzi et al, 2015;Lucas, 2009, 2010)), as previously observed by the authors for an aqueous solution of ethanol . The detailed relationship between the bubble interface properties and the BSDs are a matter for ongoing and future studies.…”

“…This is interesting because the d eq -φ space is determined by the properties at the bubble interface (the reader may refer, for example, to ref. (Firouzi et al, 2015): despite the fact that the topic is different, the framework is similar, as previously discussed). This result, taking into account that the "dual effect of viscosity over BSDs", supports the hypothesis that the stabilization of the homogeneous flow regime is caused by the large number of small "non- G. Besagni et al Chemical Engineering Science 158 (2017) 509-538 coalescence-induced bubbles".…”

Two-Phase Bubble Columns: A Comprehensive Review

“…Indeed, it is known that most electrolytes inhibit bubble coalescence in pure liquid phases [63][64][65][66][67][68] and, as a consequence, the homogeneous flow regime is stabilized [69,70]. For example, owing to their influence, the prevailing flow regime may change from heterogeneous to homogeneous, due to surfactant addition [71] as the boundary between the flow regimes changes.…”

“…Influence of the Active Compounds: Inorganic Compounds ("positive surfactants") Inorganic compounds (i.e., electrolyte solutions) are attracted to the interface where they adsorb positively, lower the surface tension, cause coalescence suppression and, finally, stabilize the homogeneous flow regime [63][64][65][66][67][68]. Indeed, it is known that most electrolytes inhibit bubble coalescence in pure liquid phases [63][64][65][66][67][68] and, as a consequence, the homogeneous flow regime is stabilized [69,70].…”

Influence of Surfactants on the Motion of a Pair of Bubbles in Quiescent Liquids

“…気泡流には様々なスケールがあり、気泡が生成 する乱れは多くの研究者の注目を集めている。そ の乱れは、流れ場と相互作用し、単相流とは異な る複雑な流動構造を示す [1][2][3][4][5][6]。特に気泡クラスタ ーは、乱流の秩序渦構造よりも大きく、大規模な 流動構造そのものを変化させる [7,8]。 そのため気 泡クラスター形成の有無について様々な報告が 行われている [9][10][11]。 気泡のクラスター形成は、まず理論的に予測さ れた。ポテンシャル流れを仮定し、複数の気泡が 上昇する際、気泡間相互作用によって水平面に気 泡 が クラスターを 形成することが報告された [12,13]。一方で、実際の気泡流では、気泡同士の 合体によって気泡径差が生まれ、通常気泡のクラ スター構造は観察されない。さらに前述のポテン シャル流れを用いた理論でも、気泡径差を考慮す るとクラスターは形成されない [14]。しかし、実 験的研究において、界面活性剤や電解質を混入さ せることで気泡クラスターの形成が報告されて いる [15,16]。 界面活性剤や電解質の混入は、気泡表面の境界 条件を大きく変化させる。界面活性剤では気泡の 抗力の増加や合体を防ぎ、電解質ではやクリーン な気泡を保ちながら気泡合体を防ぐ [17][18][19][20] /01 02 0/%/ü ! 1 ö 3 4 5 6 7 8 9 6 % !ú ÷ ù ' ü ü ö !…”