Mixing ribbon mixer

For most rapid mixing, in addition to diffusive (fine-scale) mixing, there should be a means by which large groups of particles are intermixed. This can be accomphshed by either the convective or the shear mechanism. A ribbon mixer illustrates the former, whereas a plain tumbler gives the latter. 

A final mixing system worthy of mention is that used by the Canadian Armament Research and Development Establishment and Bristol-Aerojet, which is a horizontal ribbon mixer. In this case, the blades carry the material from the center to the ends of the mixer and return it by internal spirals along the center shaft. This mixer is noted for its rapid incorporation of solids and the ease by which material is discharged as compared with the other fixed bowl, batch mixers. A comparison of typical batch mixers is shown in Table II. 

Based on the above relation, for most practical operations, this reactor behaves much hke the plug-flow reactor, where ReF = pdtL/pOT, ReM = pdiNjp. Here, 0T is the average residence time. The relationship between power consumption and mixing time reveals the similarity of this vessel to the double helical-ribbon mixer. The fraction of dead space in this apparatus appears to be small. The relationships described above are valid for the fluids in the viscosity range of 50-5000 poises. 

Numerous equipment types can be used for agglomeration, including horizontal ribbon and vertical mixers, rotary drums, zig-zag mixers, and pan agglomerators. Horizontal ribbon mixers as shown in Fig. 36.41 are used primarily for dry mix formulating, where only limited agglomeration occurs. They are comprised of a U-shaped trough and a ribbon or paddle-type agitator. Liquids are sprayed onto the surface of the dry material and slowly dispersed into the mix. Mixer operations can be either batch or continuous. 

Convective mixing predominates in machines utilizing a mixing element moving in a stationary container, for example, the horizontal ribbon mixer. Groups of adjacent particles are moved from one position to another, steadily decreasing the scale of segregation. 

Helical ribbon mixers can also be formed to fit in conical bottom tanks. While not as effective at mixing as in a cylindrical tank, the conical bottom mixer can force material to the bottom discharge. By more effectively discharging, a higher yield of the product can be obtained. 

As with open impellers, elastic fluids are apparently more difficult to process in helical ribbon mixers. To quantify this effect, the mixing efficiency of a highly elastic Separan solution is only 20 to 40% that of glycerol, which is Newtonian. 

In a ribbon mixer or V-blender, charge the puffed borax and first portion of sodium bicarbonate. Start mixing and slowly add the fragrance. After all of the fragrance has been addedj continue mixing for 5 minutes to complete absorption. Add the Larostat 519 and the remaining sodium bicarbonate. Mix for 15 minutes more. 

For very viscous liquids the helical-ribbon mixer gives a much smaller mixing time than a turbine for the same power/unit volume (M5). For nonviscous liquids, however, it gives longer times. 

Another class of devices for solids blending is the stationary shell device, in which the container is stationary and the material displacement is accomplished by single or multiple rotating inner devices. In the ribbon mixer in Fig. 3.4-6b, a shaft with two open helical screws numbers 1 and 2 attached to it rotates. One screw is left-handed and one right-handed. As the shaft rotates, sections of powder move in opposite directions and mixing occurs. Other types of internal rotating devices are available for special situations (PI). Also, in some devices both the shell and the internal device rotate. 

Materials introduced into the mixing unit (mixer or blender) are shifted in many ways by means of a fluidizing gas (fluidized bed mixer or pneumatic mixer), by mixer rotation (tumble blenders) or by the movement of some internal mixing mechanisms (ribbon, Z-blades, etc.) of the mixer (mechanical mixer). 

The literature on mixing in ribbon mixers is not extensive. The following rules can be extracted from current literature ... 

---