General Objectives Of Mixing

AND MIXING EFFECTIVENESS

Jones Industrial Mixers work closely with clients to define the mixing objectives in any plant or unit mixer. Broadly stated objectives are summarized as the “mixing of products A and B (or more products) in batch sizes of C litres capacity, in D hours per total mixing cycle, such that a final mixed product of E standard is achieved. More often than not E is loosely defined and is more often defined as low, medium or high mixing intensity.

MAIN PARAMETERS OF MIXING PERFORMANCE

The performance of an industrial mixer is judged by the time required, the power load, and the properties of the product. Both the requirements of the mixing device and the properties desired in the mixed material vary widely from one problem to another. Sometimes a very high degree of uniformity is required; sometimes a rapid mixing action; sometimes a minimum amount of power. Because of the lack of quantitative measures of mixing performance, highly arbitrary criteria of performance have been the rule, especially with mixers for pastes, plastic solids, and dry powders.

With thin liquids the amount of power consumed by the impeller per unit volume of liquid has been used as a measure of mixing effectiveness, based on the reasoning that increased amounts of power mean a higher degree of turbulence and a higher degree of turbulence means better mixing. Studies have shown this to be at least roughly true. In a given mixer the amount of power consumed can be directly related to the rate of solution of a gas or the rate of certain reactions, such as oxidations, that depend on the intimacy of contact of one phase with another. In a rough qualitative way it may be said that ½ to 1 hp per 1,000 gal of thin liquid gives “mild” agitation, 2 to 3 hp per 1,000 gal gives ‘vigorous’ agitation, and 4 to 6 hp per 1,000 gal gives “intense” agitation. These figures refer to the power that is actually delivered to the liquid and do not include power used in driving gear-reduction units or in turning the agitator shaft in bearings and stuffing boxes.

It should be remembered also that there is not necessarily any direct relation between power consumed and amount or degree of mixing. The energy delivered by any mixing device to the mixer contents is used partly for blending and partly for merely lifting the material or moving it about. Under some conditions with a thin liquid, as when it is swirled about in an un-baffled vessel, individual particles may follow parallel circular paths almost indefinitely and mix very slowly or not at all. Almost none of the energy supplied is used for mixing. The same liquid in a baffled vessel, however, may be effectively mixed by the same agitator. In the baffled vessel most of the power is utilized for mixing, and much less of it is wasted.

Mixers for pastes and plastic mixers (typically z-blades, kneaders, dispersers and masticators) operate by direct action on the material adjacent to the mixing elements. They do not set up flow currents or mix at a distance, as do impeller mixers. Instead the agitator travels throughout the mass of material, or the material is brought to the agitator. Power consumption and the amount or degree of mixing lead to complex quantitative measures and the highly arbitrary criteria of mixing performance mentioned above.

This summary is concluded by noting the wide variance of performance objectives and the need for a clear dialogue with Jones Industrial Mixers for your mixer design. Please consult with us for a detailed scoping of your mixer performance objectives.

Jones Industrial Mixers
Reference: Unit Operations of Chemical Engineering. Warren L. McCabe, Julian C. Smith

These guidelines and notes are presented as general guides only and no warranty is implied or provided.