Mixing of Multi-Phase Systems

The integral length scales or macro scales of turbulence are generally of the same order of magnitude as the impeller blade width. In addition, the macro scale of turbulence is found to scale with the impeller size. This means that for a large scale vessel, larger eddies will form than in a small vessel. Consequently, large variations in velocities and concentrations can be expected in the vessel upon scale-up. 

The previous sections describe how mixing is accomplished in a liquid phase. However, many industrial processes carried out in stirred tank reactors involve mixing of solids, gases and other liquids in a continuous liquid phase. The presence of a second phase will affect both the power consumption and the flow pattern in the tank. In the sequel, the mixing phenomena caused by the presence of gas bubbles, liquid droplets and solid particles are discussed. 

To disperse gas, the gas is usually injected into the liquid from the bottom of the tank or near the impeller to enhance dispersion. Disk style turbines are found to be most convenient for gas dispersion because the disk disturbs the freely rising gas bubbles. The turbines with flat blades give radial flow and are very useful for gas dispersion where the gas is introduced just below the impeller at its axis and drawn up to the blades and chopped into fine bubbles. 

With axial flow impellers, the upward flowing gas bubbles are allowed to rise near the shaft where the shear forces are small, so that the flow pattern and pumping effect is disrupted. Therefore, axial flow impellers are not very useful for gas dispersion [65]. 

How gas disperses in the liquid phase depends primarily on the impeller speed and the gas injection rate. The different flow regimes that may occur are illustrated in Fig 7.12. 

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Speed-up of mixing is known not only for mixing of miscible liquids, but also for multi-phase systems the mass-transfer efficiency can be improved. As an example, for a gas/liquid micro reactor, a mini packed-bed, values of the mass-transfer coefficient K a were determined to be 5-15 s [2]. This is two orders of magnitude larger than for typical conventional reactors having K a of 0.01-0.08 s . Using the same reactor filled with 50 pm catalyst particles for gas/Hquid/solid reactions, a 100-fold increase in the surface-to-volume ratio compared with the dimensions of laboratory trickle-bed catalyst particles (4-8 mm) is foimd. 

Mixing Calorimetry Instantaneous heat of mixing, AHMI)aNG Gas generation rates Isothermal, from ambient to 150°C Cannot test multi-phase systems... 

Through the understanding of the nonequilibrium changes in the glassy state of miscible blends, the excess volume of mixture is analyzed, and is related to the nonequilibrium enthalpy of mixing. In contrast to the multi-phase systems, the presence of a maximum yield stress in a miscible glassy blend at a critical concentration is predicted as a function of the nonequilibrium interaction. In accordance with Eq. (59), the total volume of a compatible blend is written as... 

Selective intensification of mass transfer with respect to reaction. With a similar scale dependence to heat transfer, one can preferentially improve the selectivity of competing reactions, in either single- or multi-phase systems. For reasons of readability, the mixing times are not discussed in this chapter but would enter this category. 

One of the important families of blended TPEs is thermoplastic natural rubber (TPNR), based primarily on polyolefin and natural rubber (NR). TPNR blends have attracted much attention due to their ease of processability and a broad spectrum of properties available at a competitive price. The TPNR system consists of multi-phases of hard and soft domains from blending thermoplastics with NR. Generally, TPNR is prepared with various type of polyolefin, such as PP, LDPE, LLDPE and HDPE. TPNR is usually prepared by a melt mixing process using an internal mixer or twin-screw extruder. It presents a wide range of useful properties, excellent processing abilities, and can be produced at moderate cost. 

Polymer blending is a useful and attractive approach to new polymeric materials with more balanced properties than their components. Most polymers are immiscible so that during the processing these compraient polymers form a multi-phase system in their blends. Several factors such as the composition, viscosity ratio, elasticity ratio, interfacial tension, shear rate and mixing time have their own influences on the final morphology of the phases. 

First, the criteria for phase equilibria are discussed in terms of single-component systems. Then, when the ground rules are in place, multi-component systems are discussed in terms of partition, distillation and mixing. 

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