Showing papers on "Mass transfer coefficient published in 2018"

Liquid–liquid two-phase flow in ultrasonic microreactors: Cavitation, emulsification, and mass transfer enhancement

Abstract: The effects of ultrasound on the hydrodynamic and mass transfer behaviors of immiscible liquid-liquid two-phase flow were investigated in a domestic ultrasonic microreactor. Under ultrasonic irradiation, cavitation bubble was generated and underwent violent oscillation. Emulsification of immiscible phases was initiated by virtue of oscillating bubbles shuttling through the water/oil interface. The pressure drop was found to decrease with increasing ultrasound power, with a maximum decrement ratio of 12% obtained at power 30 W. The mass transfer behavior was characterized by extraction of Rhodamine B from water to 1-octanol. An enhancement factor of 1.3-2.2 on the overall mass transfer coefficient was achieved under sonication. The mass transfer performance was comparable to passive microreactor at similar energy dissipation rate (61-184 W/kg). The extraction equilibrium was reached under a total flow velocity 0.01 m/s and input power 20 W and 30 W, exhibiting its potential use in liquid-liquid extraction process. This article is protected by copyright. All rights reserved.

Modelling the mid-infrared drying of sweet potato: kinetics, mass and heat transfer parameters, and energy consumption

Abstract: This study investigated the drying kinetics, mass and heat transfer characteristics of sweet potato slices (0.4–0.6 cm thickness) during drying based on mid-infrared experimental set-up (intensity of 1100–1400 W/m2). Thin layer drying models were used to evaluate the drying kinetics of sweet potato slices. Two analytical models (Fick’s diffusion model, and Dincer and Dost model) were used to study the mass transfer behaviour of sweet potato slices with and without shrinkage during mid-infrared drying. The heat transfer flux between the emitter and sweet potato slices was also investigated. Results demonstrated that an increase in infrared intensity from 1100 W/m2 to 1400 W/m2 resulted in increased in average radiation heat flux by 3.4 times and a 15% reduction in the overall drying time. The two-term exponential model was found to be the best in predicting the drying kinetics of sweet potato slices during mid-infrared drying. The specific heat consumption varied from 0.91–4.82 kWh/kg. The effective moisture diffusivity with and without shrinkage using the Fick’s diffusion model varied from 2.632 × 10−9 to 1.596 × 10−8 m2/s, and 1.24 × 10−8 to 2.4 × 10−8 m2/s using Dincer and Dost model, respectively. The obtained values of mass transfer coefficient, Biot number and activation energy varied from 5.99 × 10−6 to 1.17 × 10−5 m/s, 0.53 to 2.62, and 12.83 kJ/mol to 34.64 kJ/mol, respectively. The values obtained for Biot number implied the existence of simultaneous internal and external resistances. The findings further explained that mid-infrared intensity of 1100 W/m2 did not significantly affect the quality of sweet potato during drying, demonstrating a great potential of applying low intensity mid-infrared radiation in the drying of agricultural crops.

Vacuum drying of apples (cv. Golden Delicious): drying characteristics, thermodynamic properties, and mass transfer parameters

Abstract: In this work, apples of cv. Golden Delicious were cut into slices that were 5 and 7 mm thick and then vacuum dried at 50, 60 and 70 °C and pressure of 0.02 bar. The thin layer model drying kinetics was studied, and mass transfer properties, specifically effective moisture diffusivity and convective mass transfer coefficient, were evaluated using the Fick’s equation of diffusion. Also, thermodynamic parameters of the process, i.e. enthalpy (ΔH), entropy (ΔS) and Gibbs free energy (ΔG), were determined. Colour properties were evaluated as one of the important indicators of food quality and marketability. Determination of mass transfer parameters and thermodynamic properties of vacuum dried apple slices has not been discussed much in the literature. In conclusion, the Nadi’s model fitted best the observed data that represent the drying process. Thermodynamic properties were determined based on the dependence of the drying constant of the Henderson and Pabis model on temperature, and it was concluded that the variation in drying kinetics depends on the energy contribution of the surrounding environment. The enthalpy and entropy diminished, while the Gibbs free energy increased with the increase of the temperature of drying; therefore, it was possible to verify that variation in the diffusion process in the apple during drying depends on energetic contributions of the environment. The obtained results showed that diffusivity increased for 69%, while the mass transfer coefficient increase was even higher, 75%, at the variation of temperature of 20 °C. The increase in the dimensionless Biot number was 20%.

Simultaneous absorption–oxidation of nitric oxide and sulfur dioxide using ammonium persulfate synergistically activated by UV-light and heat

Abstract: Simultaneous absorption–oxidation process of nitric oxide (NO) and sulfur dioxide (SO2) from flue gas using aqueous ammonium persulfate synergistically activated by UV-light and heat in a novel UV-impinging stream reactor was developed. The main influencing factors, products and active species of NO and SO2 simultaneous removal were studied. The mass transfer and kinetics of NO absorption–oxidation were also investigated. The present studies show that SO2 is very easy to remove because of its very high solubility in water (SO2 removal efficiencies reach 100% under most of experimental conditions). Increasing S2O82− concentration, UV radiation power, activation temperature or liquid–gas ratio promotes absorption–oxidation of NO. Increasing solution pH, concentrations of NO and SO2 or flue gas flow inhibits absorption–oxidation of NO. The contents of O2 and CO2 have no significant effect on removal of NO. The highest simultaneous removal efficiency for NO and SO2 is 96.1% and 100%, respectively. OH and SO4− are the key oxidizers for NO removal. Absorption–oxidation process of NO meets a fast pseudo-first order reaction for NO, and can be enhanced by increasing the gas-side mass transfer coefficient and interfacial area.

Mass Transfer Parameters for Packings: Effect of Viscosity

Abstract: The effect of liquid viscosity (μL) on the effective mass transfer area (ae) and liquid film mass transfer coefficient (kL) of packing must be known to determine packing height. Most existing correlations are based on water and are not applicable to viscous systems. In this work, ae, kL, and gas film mass transfer coefficient (kG) were measured in a 0.43 m ID packed column with 0.5–3 m of packing. Liquid viscosity was varied from 0.8 to 70 mPa·s by adding glycerol to water. Liquid viscosity has an insignificant effect on ae over the range of μL investigated. The total dependence of kL on μL is −0.75, of which −0.35 is indirect influence through diffusivity, and −0.4 is direct influence through liquid turbulence. Universal mass transfer models for packing were developed as functions of packing geometry, operating condition, and fluid property.

Optimization of monoethanolamine for CO2 absorption in a microchannel reactor

Abstract: In this study, CO2 absorption was performed using a MEA aqueous solution in a circular microchannel, with a diameter of 800 μm and a length of 25 cm. All experiments were carried out at atmospheric pressure with a concentration of 10% molar CO2 in the feed gas. The operational variables were temperature (15–55 °C), input solvent flow rate (0–0.04 l min−1), inlet gas flow rate (1–9 l.min−1), and MEA concentration in the solvent (0–40 wt.%). The results showed that by increasing the concentration of MEA in the solvent, and increasing the solvent flow rate, a significant increase occurred in the absorption percentage and overall gas-based volumetric mass transfer coefficient. Increasing the gas flow rate also caused a reduction in the absorption percentage and an increase in the overall gas-based volumetric mass transfer coefficient. However, the operating temperature within a range of 15–55 °C did not have a significant impact on absorption rate. In optimal operating conditions, the absorption percentage was 100%, and the overall gas-based volumetric mass transfer coefficient of 2685 (kmol h−1 m-3 kPa−1) was reached. The overall gas-based volumetric mass transfer coefficient indicates that, compared with other mass transfer devices, the use of a microchannel-reactor resulted in a higher level of increase in absorption efficiency.

Removal of SO2 with Sodium Sulfite Solution in a Rotating Packed Bed

Abstract: High efficiency and compact design of the desulfurization technology is necessary to meet the strict emission requirement of the marine environment. A rotating packed bed (RPB), having the high mass transfer efficiency and small size, is an excellent choice for SO2 removal from the flue gas in ships or warships. In this work, SO2 removal in a laboratory and a pilot RPB with sodium sulfite solution was investigated. Experimental results show that the SO2 concentration at the gas outlet of the RPB can meet the emission requirement and it has no obvious scale up effect. A correlation of the mass transfer coefficient (Kya) was proposed and the predicted Kya agrees with the experimental data with a deviation within ±12%. Compared with the spray tower, the RPB has a higher SO2 removal efficiency and a quarter volume, which displays a bright future for the offshore application of SO2 removal.

CO2 Capture by Alkaline Solution for Carbonate Production: A Comparison between a Packed Column and a Membrane Contactor

Abstract: A comparison between a traditional packed column and a novel membrane contactor used for CO2 absorption with carbonate production is addressed in this paper. Membrane technology is generally characterized by a lower energy consumption, it offers an independent control of gas and liquid streams, a known interfacial area and avoids solvent dragging. Those advantages make it a potential substitute of conventional absorption towers. The effect of the concentration and the flow rates of both the flue gas (10–15% of CO2) and the alkaline sorbent (NaOH, NaOH/Na2CO3) on the variation of the species present in the system, the mass transfer coefficient, and the CO2 removal efficiency was evaluated. Under the studied operation conditions, the membrane contactor showed very competitive results with the conventional absorption column, even though the highest mass transfer coefficient was found in the latter technology. In addition, the membrane contactor offers an intensification factor higher than five due to its compactness and modular character.