Mixers axial mixing

When the mixer s task is primarily blending of different constituents and dampening of feed irregularities, an agitator that causes a fair amount of axial mixing is desirable. In simple twin-screw extruders, this may be achieved by... 

Distributive (radial) mixing is usually excellent dispersive (axial) mixing is often poor. The result can be a good plug-flow mixer or reactor, with corresponding benefits and limitations. 

FIG. 21-55 The continuous mixing of two ingredients Axial mixing or dispersion shows up as well as residence time distribution of the product inside the mixer. 

Impellers and paddle are larger slower-speed mixers with two or four blades. Both mixer types are almost universal for low and moderate viscosity. The flat-blade turbine has a predominant radial action being suited for turbulent mixing, as in the case of gas-liquid dispersions. The pitched-blade mixer and the marine propeller are appropriate for axial mixing. 

There are a number of potential problems associated with the use of static mixers °. An important limitation, which is not very obvious, is that the virtual absence of any axial mixing requires that the feed streams are held at a constant flow ratio—not always easy to achieve in practice. For some applications, e.g. liquid-liquid and gas-liquid dispersions, mixer performance will depend on flow rate and there will be a minimum value below which a dispersion will not be created. 

Initially, a major advantage of the micro-reactor was its ability to achieve effective mixing this was due to the small axial dimensions and, hence, small diffusion distances [1]. Nonetheless, later versions of micro-reactors were often preceded by a micro-mixer, which mixed the reaction components very effectively and quickly. The exception to this was the preparation of so-called Janus particles in a micro-reactor [2, 3], when it was important to prevent any mixing. Although a simple T-junction can serve as a micro-mixer, more advanced mixing devices are now available on a commercial scale (Figure 14.1). 

This section will analyze how the velocity profiles, axial mixing, and residence time distribution are related. It will be shown why simple conveying screws have poor axial mixing capability. New mixer geometries that are specifically designed to improve backmixing will be discussed. 

The challenge in improving axial mixing is to efficiently transfer fluid from the inner re-circulation region to the outer region and vice versa. A simple but effective method of doing this is the inside-out mixer shown in Fig. 7.161. 

Treybtd Extractor [73] This is in reality a vertical stack of mixer-settlers. The mixers are in a vertical line, and the impellers for the stages on a common shaft. They not only mix but also pump, so that throu put rates are high. Since there is no axial mixing, mass-transfer rates are high. 

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