Sizing your horizontal batch mixer
In batch mixing, ingredients are loaded into the mixer, the mixer is operated, and the mixture is unloaded so another batch can be loaded into the vessel. This operation is quite different than continuous mixing: In a continuous mixer, ingredients flow steadily from upstream processing into the operating mixer and are retained and mixed for a specified time, and the mixture discharges steadily from the mixer at the same flowrate. Because of this operating difference, a batch mixer is often used for mixing smaller quantities of material, handling multiple ingredients or multiple product formulas, and providing longer mixing times, such as to blend difficult-to-mix ingredients.
Of the several machines that provide batch mixing, a horizontal batch mixer is one of the most popular in bulk solids plants. Two common types are ribbon and paddle mixers. Each consists of a stationary U-shaped trough (or tank) equipped with a rotating agitator shaft. The shaft in a ribbon mixer, as shown in Figure 1, is fitted with metal spokes (or cross arms) and helical blades called ribbons;
in a paddle mixer the shaft is fitted with paddle-shaped blades. As the shaft rotates, the ribbons or paddles move and agitate the material to create a mixing action. In a variation of this equipment, called a plow mixer,
the rotating agitator shaft has plow-shaped blades and the housing can be cylindrical rather than U-shaped.
Determining the optimal size for your batch mixer is based on the working volume, fill level, and the residence time your mixing process requires.
is the volume of material to be mixed and can be determined by dividing your batch size (in cubic feet) by the bulk density (in pounds per square foot) of your ingredients. The supplier can use the batch size and bulk density information on the application data sheet you filled out when selecting the mixer to help you calculate the mixer's working volume.
Fill level is the allowable batch size, which is expressed as a percentage of the mixer's total capacity. Loading a mixer to less than 100 percent of its total capacity provides room for the material to move within the mixer vessel, which enhances mixing. The optimal fill level depends on the mixer type and represents the ideal compromise between using the smallest possible mixer and achieving the greatest interaction between the material and its agitator, called agitator engagement.
In batch ribbon and paddle mixers, the higher the mixer's fill level, the more passive mixing occurs — that is mixing in areas not in contact with the agitator, which form the passive mixing zone,
as shown in Figure 2. In these mixers, as the fill level exceeds 50 percent and rises above the agitator's middle, mixing efficiency drops because more material fails to contact the agitator and instead enters the passive mixing zone. Material in this zone not only takes longer to mix, but may never become completely mixed. However, using a fill level above 50 percent may be acceptable in an application where the mixing specification is relatively loose (such as for some animal feeds) or where the user has plenty of time in the process for a longer mixing cycle and wants to use a smaller mixer because of its reduced cost.
The mixer supplier can typically provide mixers designed to work at different fill levels. Based on experience with hundreds of mixing applications, the supplier can recommend a practical fill level for your new horizontal batch mixer. When requesting bids from multiple suppliers to replace an existing mixer operating at a particular fill level, ask them all to specify a mixer of the same size operating at the same fill level so you're comparing apples to apples.
Residence time is the length of time material is in the mixer. In some cases, residence time expresses the time required to distribute all the ingredients throughout the mixer. In others, it expresses the time required to complete a chemical reaction in the mixer, such as the powder ingredients' absorption of a small amount of added water to form a smooth, consistent mixture without clumps.