Convective mass exchange between flows in a valve-pulsatory apparatus with permeable partition

A mathematical model of convective mass exchange between flows in a valve-pulsatory apparatus with permeable partition is proposed. The mass exchange between flows of aqueous Na₂CO₃ solution and distilled water, or pearlite suspension in water and aqueous solution of NaCl was studied on a pilot apparatus.     

Continuous production of solid lipid nanoparticles by liquid flow-focusing and gas displacing method in microchannels

The liquid flow-focusing and gas displacing method is developed to produce solid lipid nanoparticles (SLNs) continuously in a microchannel, which has a cross-junction for the focus of lipid and aqueous solutions and a T-junction for the injection of gas bubbles. The liquid flow-focusing was achieved by introducing a lipid solution with a water-miscible organic solvent and an aqueous surfactant solution simultaneously through the two branches of the cross-junction into the main channel, while the gas displacing was accomplished by injecting an inert gas (N₂) through the T-junction at the downstream of the cross-junction into the main flow streams upward to form gas-liquid slug flow. Solid lipid nanoparticles were formed due to the local supersaturation of lipid induced by the diffusion of the solvent from the lipid solution stream into the aqueous phase. The liquid suspension containing solid lipid nanoparticles then passed freely through the microchannel without any blockage by the contribution of gas slug flow. The flow behaviors were observed by a digital inversion microscope system and the hydrodynamics of the liquid flow-focusing streams and the gas slug flow were investigated. Particle size distributions of the solid lipid nanoparticles obtained under various conditions were measured by dynamic light scattering and the particle morphology was examined by transmission electron microscopy. The influences of liquid velocity and lipid concentration under the gas displacing condition on the properties of solid lipid nanoparticles were studied experimentally. The solid lipid nanoparticles with small size (the mean size in the range of 120-200 nm) and narrow particle size distribution (with values of polydispersity index in the range of 0.14-0.19) had been produced by this method. The crucial roles of Taylor bubbles and liquid slugs in the formation of solid lipid nanoparticles were considered and the transfer mechanism of slug flow on the formation and passage of solid lipid nanoparticles in the microchannel were also discussed. Compared with other production methods for SLNs (e.g., hot homogenization, warm microemulsions and supercritical fluid technique), the proposed method in this work is simple and no overcritical operations are needed during the preparing process. Therefore, it can be employed to prepare SLNs with small sizes and a narrow diameter distribution.     

Effect of transfer in the vapour phase on the extraction by pervaporation through organophilic membranes: experimental analysis on model solutions and theoretical extrapolation

Mass transfer in pervaporation is usually regarded as limited by the solution-diffusion step inside the dense selective polymer layer. In the case of pervaporation for the extraction of volatile organic compounds through organophilic membranes, especially at low feed temperature (about 300 K), the influence of the downstream pressure cannot be neglected. A contribution to the study of the operating parameters on the vapour side in a pilot plant — from the membrane to the condenser — to the overall mass transfer is presented.

A “convection-diffusion” model has been established to calculate the partial pressure gradients in the vapour phase up to the downstream face of the membrane. This equation has been combined with a relation for the mass transfer inside the membrane with a driving force expressed as a difference in fugacities.

The partial permeate pressures and the pervaporate fluxes obtained first with a pure compound (water) and secondly with binary mixtures (water-ethanol) pervaporated through membranes of polydimethylsiloxane (PDMS) on a pilot plant scale are well predicted by the model. Moreover, on the permeate side, the effects of unavoidable non-condensable gases, of the condenser temperature and of the distance between the module and the condenser on the flux and on the selectivity have been established for different total permeate pressures (300–3000 Pa). At high pressure, the pervaporation selectivity towards ethanol exhibits a minimum value as a function of the permeate circuit design.     

加热方式对管壳式相变蓄热单元强化传热的可视化及数值模拟

为了提高相变蓄热系统在实际应用中的蓄热速率,本文建立了管壳式相变蓄热单元可视化实验平台,提出了一种外部加热法的强化传热方式,讨论了相变蓄热单元在外部加热法时的熔化特性和传热机理。通过Fluent软件模拟对比了外部加热法和内部加热法在熔化过程中的液相分数、熔化速率和均匀性等方面的差异。研究结果表明：外部加热法的使用能大幅提升蓄热单元的蓄热效率。与内部加热法相比,外部加热法在熔化过程中的传热和相变更加均匀。相比于内部加热法,由于外部加热法传热面积较大,熔化时间缩短了69.1%;在消除传热面积的影响后,外加热方法依靠广泛而强烈的自然对流使熔化时间减少了23.2%。     

Measurement of the local velocity of the solid phase and the local solid hold-up in a three-phase flow by X-ray based particle tracking velocimetry (XPTV)

The measurement of the local solid velocity and the local solid hold-up in three-phase flows (gas, liquid, solid) is of great interest with regard to the design of three-phase reactors. Moreover, such measurements are necessary for the validation of flow simulations of three-phase flows. The optical methods usually applied for velocity measurements such as particle image velocimetry do not work in three-phase flows. This is due to the opacity of the solid phase or/and because of the reflections and refractions that occur on phase boundaries. Other measurement methods are intrusive and very time consuming. The measurement of the local solid hold-up is even more difficult. The new X-ray based particle tracking velocimetry (XPTV), described in this paper, measures the solid velocity and the solid content simultaneously. This fast working and non-intrusive technique has already been successfully applied in a bubble column to measure the liquid velocity. XPTV is a three-dimensional three-component method. It works independently from void fraction and solid hold-up.