Gas-Liquid Direct-Contact Evaporation: A Review

Abstract

Gas-liquid direct-contact evaporators are characterized by the bubbling of a superheated gas through the solution to be concentrated. In other words, they are nonisothermal bubble columns. Despite their simplicity of construction, these units exhibit rather complex hydrodynamics and, similar to what occurs to isothermal bubble columns, the design of such units still poses a problem. The present paper reviews the literature regarding this kind of equipment, addressing both experimental studies and modeling efforts. The covered issues include classic and potential applications, bubbling regimes, gas holdup and bubble size distributions, as well as mathematical models proposed for simulating the unit. Additionally, pertinent literature on isothermal bubble columns is also discussed. Recommendations are made for future research.

On the estimation of the regime transition point in bubble columns

“…This number is surprisingly small if one considers that, in a recent review [4], more than 30 correlations were listed for the mass-transfer coefficient in the liquid phase and almost 40 correlations were given for the gas hold-up in this kind of gas-liquid contactor.…”

“…(10) is written in terms of independent variables that are physically related to bubble formation and coalescence phenomena. The Morton number is the standard dimensionless parameter to represent the effect of the physical properties of the liquid phase, found, for instance, in many correlations for gas hold-up in bubble columns [4]. Reynolds and Weber numbers, on the other hand, have already been adopted to understand and correlate the critical velocity for bubble coalescence [61][62][63][64].…”

“…This is quite a challenging task, since hydrodynamics are fully coupled with breakage and coalescence frequencies. A few approximate flow regime maps are available, but none of them covers a wide range of industrial conditions [11]. Some theoretical transition criteria have even been developed [8,[11][12][13], but, as detailed by Ribeiro and Lage [4], they all have limitations. In the past two decades, a considerable effort has been made to represent and understand the complex hydrodynamics of bubble columns with the aid of computational fluid dynamics (CFD) for a constant bubble diameter [14,15].…”

“…Depending primarily on the gas superficial velocity, u G , a bubble column can basically operate under three different regimes [3,4]. For low u G values, small, uniform-sized bubbles are observed, whose ascension trajectories are practically ver- * Tel.…”

Hydrodynamic Considerations in Bioreactor Selection and Design

“…The batch bubble column is the more common form, but the cocurrent version, shown in Figure 4, is also encountered. Countercurrent liquid flow is rarely used in industry as it provides minor, if any, advantages and multiple complications [32], with separation by evaporation being one of the few exceptions [33]. Bubble columns tend to be ta ll ratio (H/DR) because the he ight i a process and residence time, especially ~ r at hand operations [34].…”