Mixing (process engineering)

Abstract: Liquid films falling over banks of internally cooled horizontal tubes are often used to absorb mass from a surrounding vapor This arrangement is particularly suitable for absorption processes where the vapor has a high heat of absorption and where high transfer rates and low pressure drops are required, as is the case of absorption heat pumps and other chemical processes When the liquid film presents a significant resistance to heat and mass transfer, understanding the motion of the film is critical However, mathematical models of these types of systems in the literature have generally made use of many simplifying assumptions about the behavior of the falling liquid The formation, detachment, and impact of droplets and the associated waves and film disturbances can all affect the mixing of the liquid and can enhance transfer rates accordingly The objective of this paper is to identify and visually document these deviations from idealized film behavior and discuss their implications on the heat and mass transfer processes, which are important to consider in the development of mechanistic models of the absorption process

Abstract: [1] The success of in situ bioremediation projects depends on the mixing of contaminants and nutrients in the presence of microbes. In this work, a pore-scale model is developed to simulate biomass growth that is controlled by the mixing of an electron donor and acceptor. A homogeneous packing of cylinders representing solid grains is used as the model two-dimensional porous medium. The system is initially seeded with microbes in computational cells located at grain-water interfaces. The solutes enter the system completely unmixed; each solute is input over one half of the inlet boundary. Solute mixing is controlled by molecular diffusion transverse to the flow direction, and solutes are biotransformed according to dual Monod kinetics only where biomass is present. Simulation of biomass growth requires calculation of the water flow field as well as transport and reaction of solutes. The lattice Boltzmann method is used to obtain the flow field. Transport and reaction of the solutes is modeled by a finite volume discretization of the advection-diffusion-reaction equation. Biomass is allowed to grow and spread by means of a cellular automata algorithm. Model parameters are systematically varied to understand their effects on biomass development. Base case parameter values are obtained from batch experiments reported in the literature and are modified to achieve agreement between simulation results and previously reported micromodel experimental results. The most significant mechanisms that control biomass development are shear strength of new biomass and solute degradation rates. The biomass growth model achieves good qualitative agreement with experimental results.

Abstract: A CONTINOUS PREPARATION OF THERMOPLASTIC MONOCELLULAR POLYURETHANES IS DISCLOSED. THE REACTION COMPONENT ARE MIXED IN THE LIQUID STATE AND PASSED THROUGH A HIGH SHEAR MIXING ZONE AND THEN THROUGH ANDEXTRUSION ZONE. THE TEMPERATURE OF THE REACTION MIXTURE IS CONTOLLED DURING PASSAGE THROUGH THE MIXER AND EXTRUDER IN SUCH A MANNER THAT THE VISCOSITY OF THE REACTION MIXTURE REMAINS SUBSTANTIALLY CONSTANT THROUGHOUT THE MIXING AND EXTRUSION ZONES; THIS REQUIRES A TEMPERATURE GRADIENT IN THE VARIOUS ZONES RISING FROM A LOW OF ABOUT 200*F. AT MIXING TO ABOUT 400-450*F. AT THE POINT OF EXTRUSION. IN A PREFERRED EMBODIMENT THE REACTION IS CARRIED OUT IN A COMBINATION OF HIGH SHEAR MIXER OAN TWIN-SCREW EXTRUDER. THE THERMOPLASTIC POLYURETHANE IS PREFERABLY EXTRUDED AS A STRAND WHICH IS COOLED BELOW ITS MELTING POINT AND PELLETIZED.

Abstract: This study investigated solids mixing of a group A fine powder in a 0.15 m ID expanded top fluid bed with a ferromagnetic tracer. The superficial gas velocity was raised from 0.075 to 1.1 m/s, causing the bed to go through bubbling, slugging, and turbulent fluidization regimes. A countercurrent flow model described the data well at low gas velocities. The bed assumed a more homogeneous appearance at higher gas velocities; a one-dimensional axial dispersion model was used to correlate the data. Axial dispersion coefficients increased with gas velocity. The data agree well with literature data for low gas velocities.