Showing papers by "Cláudio P. Ribeiro published in 2005"

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.

Hydrodynamic Behavior of a Rotating Disc Contactor Under Low Agitation Conditions

Abstract: The hydrodynamics of a rotating disc contactor (76.2 mm diameter, 19 stages, and operating height of 815 mm) has been investigated at limiting operational conditions of low holdup (φ ≤ 3%) and low agitation (Re R ≤ 22,000) for the water/n-heptane system in the absence of mass transfer. Water was chosen as the continuous phase, and it was kept stagnant inside the column. Local drop size distribution, local mean drop diameter, and total holdup have been measured at different flow rates of the dispersed phase (70 to 640 mL/min) and rotor speeds (0 to 700 rpm). Drop size distributions along the column were suitably described using the log-normal probability density function, and empirical expressions were derived to predict the parameters of the distribution curve as a function of operating conditions. An empirical expression was also proposed to predict the Sauter mean drop diameter along the column with a mean deviation of 3.4%. Statistical analysis has revealed no significant effect of the rotor ...

A New Route Combining Direct-Contact Evaporation and Vapor Permeation for Obtaining High-Quality Fruit Juice Concentrates. Part I: Experimental Analysis

Abstract: An alternative rote is proposed for fruit juice processing, according to which aroma compounds are extracted by gas stripping in a bubble column and then recovered by vapor permeation, in a first step, the aroma-depleted juice being subsequently concentrated by direct-contact evaporation. To illustrate the adequacy of the proposed process, experiments were conducted with a synthetic juice, an aqueous solution of sucrose and two esters. In these tests, an aroma recovery degree of about 90% was obtained. Even though the operating pressure was slightly higher than 101 kPa, the liquid temperature did not exceed 66 °C and sucrose concentrations up to 77 wt % were achieved without any reduction in the evaporation rate. Additionally, pervaporation experiments were conducted with an aqueous solution of the esters and the flux results were compared with those related to the vapor permeation tests, revealing that the latter technique is less susceptible to concentration polarization and hence more appropriate for a...

A New Route Combining Direct-Contact Evaporation and Vapor Permeation for Obtaining High-Quality Fruit Juice Concentrates. Part II: Modeling and Simulation

Abstract: A mathematical framework is presented for simulating each step of the proposed juice-processing route. Both the gas stripper and the direct-contact evaporator were simulated by combining a transient, well-mixed model for the liquid phase with a detailed model for heat and multicomponent mass transfer in superheated bubbles that was previously developed. For vapor permeation, a multicomponent, one-dimensional, plug-flow model for a hollow-fiber module was developed, considering both countercurrent and cocurrent configurations. These models were tested against experimental data. A sensitivity analysis of the proposed route was also conducted considering two main operation parameters: evaporator and permeate pressures. When the former was reduced, a significant drop in the liquid temperature in tandem with an augmentation of the evaporation rate was observed, whereas an increase in the latter led to a lower recovery degree in the membrane module.