Gas-Liquid Direct-Contact Evaporation: A Review (original) (raw)
Related papers
International Journal of Heat and Mass Transfer, 2004
Considering the bimodal feature of the bubble size distribution in the heterogeneous regime, a recently developed model for simulating direct-contact evaporators operating in the homogeneous regime was extended to the heterogeneous bubbling regime, enabling, thereby, the simulation of a direct-contact evaporator for any bubbling regime. The proposed model includes a correction factor for isothermal gas holdup correlations to account for heat and mass transfer effects which arise in non-isothermal bubbling. The model predictions were shown to be in good agreement with literature experimental data for the air-water system, considering four different gas superficial velocities.
Experimental study on bubble size distributions in a direct-contact evaporator
Brazilian Journal of Chemical Engineering, 2004
Experimental bubble size distributions and bubble mean diameters were obtained by means of a photographic technique for a direct-contact evaporator operating in the quasi-steady-state regime. Four gas superficial velocities and three different spargers were analysed for the air-water system. In order to assure the statistical significance of the determined size distributions, a minimum number of 450 bubbles was analysed for each experimental condition. Some runs were also conducted with an aqueous solution of sucrose to study the solute effect on bubble size distribution. For the lowest gas superficial velocity considered, at which the homogeneous bubbling regime is observed, the size distribution was log-normal and depended on the orifice diameter in the sparger. As the gas superficial velocity was increased, the size distribution progressively acquired a bimodal shape, regardless of the sparger employed. The presence of sucrose in the continuous phase led to coalescence hindrance.
Advances in Direct Contact Evaporator Design
Chemical Engineering & Technology, 2004
Direct contact evaporators are nonisothermal bubble columns where a hot gas, usually obtained by combustion, is used to heat and vaporize the solvent of a given solution that needs to be concentrated. The design of such equipment has been relied on experimental data or on a simplified method that assumes that the gas leaves the evaporator at equilibrium with the liquid phase, giving no information about the necessary bubbling height to attain such equilibrium conditions. Recent advances in the heat and mass transfer processes during the formation and ascension of superheated bubbles together with simple mass and energy balances in the liquid phase and gas distributor system were used to develop a more detailed design procedure. The accuracy of both design procedures are compared to available experimental data in a direct contact evaporator operating in semibatch mode. The new design method agreed well with the experimental data.
Chemical Engineering Science, 2004
The study of bubble size distributions in direct-contact evaporators was addressed both theoretically and experimentally. Recently developed models for calculating bubble coalescence and breakage frequencies in isothermal bubble columns were adapted to the population balance equation using the bubble mass as the internal coordinate which was discretized using an expansion of the number density function by impulse functions. A sparger model was developed based on experimental data for a non-coalescing system and using bubble formation models for isothermal and non-isothermal conditions. Bubble size distributions in a direct-contact evaporator operating in the quasi-steady-state regime for four di erent gas superÿcial velocities, including the homogeneous and heterogeneous regimes, together with the sparger model, were used for estimating the three empirical parameters from the population balance model, which were observed to be functions of the gas superÿcial velocity. In all cases considered, the population balance model ÿtted the experimental data rather well and the regressed parameters exhibit the physically expected behavior with changes in the gas superÿcial velocity.
Chemical Engineering Science, 2005
A recently developed model for coupled heat and mass transfer in binary systems during the formation and ascension of superheated bubbles was extended to a multicomponent system comprising N volatile species. The model allows variable properties and bubble radius changes, assuming diffusive mass fluxes to be properly described by Fick's law. Experimental direct-contact evaporation tests were conducted with ethyl acetate aqueous solutions to provide data for assessing the developed model. In addition, the model was tested against available literature data for an air-stripper. In both cases, a good agreement between simulation and experimental results was verified.
Heat transfer in three-phase fluidization and bubble-columns with high gas holdups
AIChE Journal, 1993
Bubble column and three-phase fluidized bed reactors have wide applications in biotechnological and petroleum processes (Deckwer, 1985; Fan, 1989). In such biotechnological processes as fermentation and waste water treatment, small bubbles of oxygen and/or nitrogen are introduced in the column to enhance oxygen transfer and to ensure the stability of immobilized cell particles. In addition, tiny bubbles are formed by such gases as C02, H2, and CHI, which are produced during the biological process due to the production of surface active compounds. The presence of these small bubbles causes an increase in the gas holdup of the system. High gas holdups are also characteristics of industrial processes such as coal liquefaction and hydrotreating of residual oils. Tarmy et al. (1984) studied high gas holdup behavior in pilot-and large-scale coal liquefaction reactors and reported gas holdup as high as 50 ~0 1 %. Such high gas holdups were attributed to the formation of small rigid bubbles with noncoalescing tendencies due to high-pressure and high-temperature operations. Blum and Toman (1977) reported a maximum gas holdup of 0.5 in a hightemperature (100 to 350"C, 6.9 MPa) three-phase fluidized bed methanator. Thus, good understanding of the transport properties of three-phase fluidized beds with high gas holdups is essential to the design, control and optimum operations of the commercial reactors employed in the above-mentioned processes. High gas holdups in laboratory scale under ambient conditions can be obtained by the addition of surfactants in airwater and air-water-solid systems. Such high gas holdups in the presence of a surfactant simulate the conditions commonly encountered in coal liquefaction and petroleum resids hydrotreating operations. Levich (1962) reported that the presence of surfactants induces a noncoalescing tendency in the gasliquid flow and reduces the bubble rise velocity significantly by creating surface tension gradients that cause tangential stresses along the bubble surface. The addition of surfactants to pure water systems, however, increases the system complexity by altering the average bubble size, gas holdup, and bubble-particle interactions. This causes the flow regime transition which affects the heat-transfer behavior of the system. In addition to the dependence on liquid properties, operating Correspondence concerning this work should be addressed to L.-S. Fan. conditions for flow regime transition also depend on the gas distributor design, particle properties, and the column size. Several researchers (such as Kelkar et al., 1983; Shah et al., 1985) reported the effects of surfactants on bubble column hydrodynamics. Kelkar et al. (1983) observed that the gas
Water boiling at low pressure: dynamics of growth and bursting of bubbles in a plate-type evaporator
2020
Water is a good candidate to be used as an eco-friendly refrigerant. However, its behaviour under subatmospheric pressure needs to be better understood to optimize the efficiency and sizing of compact evaporators. Previous studies conducted in a plate-type evaporator highlighted that after the bubbles burst, the formation and evaporation of a thin liquid film on the evaporator wall has a major contribution on heat transfer. This work thus aims to characterize the dynamics of vapour bubbles during nucleate boiling, and more specifically the projection height after the free surface breakup. Using previous high-speed visualizations, the bubbles’ characteristics are computed thanks to an ad-hoc Python program. It is shown that the projection height is correlated with the growth velocity and the size of the bubble reached just before it bursts. The influence of relevant dimensionless numbers on the projection height is also studied to determine the underlying predominant forces.
Heat Transfer Between Gas and Non-coalescent Liquid in a Bubble Column
Chemical engineering transactions, 2019
Multiphase contactors, e.g. bubble columns, are often used in operations accompanied by heat transfer between the phases. The effect of multi-orifice aerator for the heat transfer between gas and the noncoalescent liquid was investigated for two different aerator patterns. This study was performed on the bubble column 0.15 m in diameter with various liquid levels and for various superficial gas velocities. The aqueous solution of 0.3 M Na 2 SO 4 was used as a non-coalescent batch. The gas-to-liquid heat transfer measurements were performed by a non-steady state method based on measurements of the gas-and liquid-temperature in the time for an evaluation of the heat fluxes and heat transfer coefficient. 2. Theoretical background 2.1 Coalescence Approaching two bubbles to each other the liquid is drained from the space between bubbles and liquid film thickness decreases. If the liquid film is ruptured, the bubbles coalesce and form one bigger bubble. Many authors investigated the effect of the salt ions on bubble coalescence (e.g.
Influence of heat-exchanging tubes diameter on the gas holdup and bubble dynamics in a bubble column
Fuel, 2019
The effects of the presence of vertical internal tubes and their diameters on the local gas holdup and bubble dynamics, including the specific interfacial area, bubble chord length, and bubble velocity were investigated in a 6 in. bubble column for the air-water system by using a four-point optical fiber probe technique. Two different diameters, 0.5-inch, and 1-inch, of vertical internals equally covering 25% of the column's cross-sectional area (CSA) were used to represent the heat-exchanging tubes utilized in the Fischer Tropsch (FT) process. For both sizes, the vertical internals were uniformly distributed over column CSA. The experiments were performed using the air-water system, in a 6-inch bubble column at superficial gas velocities of 20, 30, and 45 cm/s. The experimental results indicated that the presence of vertical internals and their diameters have a significant effect on the hydrodynamic properties of the bubble column reactor at high superficial gas velocities. The local gas holdup significantly increased in the core region and decreased at the wall regions when the 0.5-inch vertical internals were used. Contrarily, the 1-inch vertical internals enhanced the gas holdup near to the wall regions. Additionally, the bubble chord length and the bubble rise velocity were found to be larger in the presence of vertical internals, especially at high superficial gas velocities. The specific interfacial area with the 0.5-inch internal was much lower than bubble column without vertical internals, but while using 1-inch internals, it was enhanced in the wall regions.
Experimental study of gas hold-up and bubble behavior in gas-liquid bubble column
Experimental reactor was a cylindrical bubble-column made of glass, with an inside diameter of 15cm and a height of 2.8(m). The column was equipped with two types of sparger, a porous plate and a perforated plate with the same porosity. In this study, liquid phase and gas phase were water and air respectively. Gas hold up, bubble size and effect of sparger type in different gas velocity were investigated. Gas hold up was determined with differential pressure method and used to estimate the transition velocity in slurry bubble column reactors. The results showed that with increasing the superficial gas velocity, the total gas hold up increases. Also perforated-type sparger increases the diameter of bubbles up to 35% and decreases gas hold up to about 40% respectively. Also it was found that the Hikita's correlation predicts the gas hold up value better than other presented correlations in this system.