Mechanical High-Shear Dispersion

By Michael Janssen | May 19, 2010
High-shear dispersion machines for liquid/liquid, solid/liquid, and liquid/gaseous have gained wide use in biodiesel laboratories and manufacturing plants. It is of broad interest to the emerging fuel industries and specifically to the manufacturers and processors of fatty acid methyl esters to utilize an economical, versatile and efficient dispersion methodology. On the laboratory scale, dispersion and formulation challenges in crude oil/fat refining such as acid or enzymatic degumming, neutralization, acid esterification, base transesterification, and even finishing processes such as dry or water wash processes, can be solved with small in-line or batch high-shear mixers.

Because of the many scientific and commercial challenges to be solved by dispersion and high-shear techniques, there is a need for an efficient system of high-shear mixers meeting the following requirements:

1. Treatment of small volumes and large quantities should be possible.

2. Precise scale-up to commercial machines from the same supplier.

3. Batch, continuous-flow, circulation and in-line methodologies should be available for laboratory and production scale.

4. Energy demand should be low.

5. The rotor/stator system should be compatible with the reactive and flammable process.

6. Flexibility by shear-frequencies and numbers in order to accommodate different processes such as enzymatic degumming (ultra high-shear) or secondary transesterification (moderate high-shear).

Today there are many mixing techniques available. Some are as old as Rudolf Diesel himself, who used a simple agitation mixing technique, and some others more recently invented during the latest biofuel hype. Newer techniques have evolved with interesting names such as shockwave, dynamic cavitation, hydrodynamic and ultrasonic.

All these mixing techniques including agitation and high-shear dispersion have the same goal: reduce as fast as possible the droplet sizes of the two or three immiscible liquid phases and accelerate the reaction. The faster that small micron droplets are formed the faster the reaction becomes complete. The common transesterification process with a common ration of 78/20/2 percent can take up to 4 hours when mixed by conventional agitators.

Using high-shear methods can reduce the batch cycle down to 30 minutes or operate continuously. Many of the new plants will be designed with fully automated continuous systems. Current batch processors are searching for ways to produce in-line by utilizing their existing equipment. Depending on location, investment, space and other requirements, batch, semi-continuous, or fully continuous process approaches are utilized.

High-shear machines or generators, which were developed in the 1950s by Prof. P. Willems in Germany, meet all the requirements for refining, reaction enhancement and washing applications, as mentioned before.

High-shear dispersers consist of a rotor inside a stator. The rotor is driven by an electric motor at speeds up to 26,000 revolutions per minute (Magic Lab High-Shear). Both the rotor and the stator have one or more, up to four, concentric rings of slots and teeth manufactured in 316 stainless or exotic alloys on request. Together the interlocking rotor/stator is termed a generator. The dimension can range from 10 millimeters (mm) up to 400mm depending on the volume or flow rate of the liquid streams to be processed. The inline machines can be fitted on one driveshaft with up to three generators (multiple stage high-shear mixer type Dispax Reactor). The shape of the teeth in the rotor and stator is chosen in such a way that the whole assembly, together with additional mechanical tools, acts like an efficient centrifugal pump. The liquid streams with or without suspended particles are pressed through the succeeding concentric rings of slots and teeth. During this torturous passage, each liquid portion is subdivided successively into very small volume portions that undergo high-frequency mechanical treatment produced by the relative movement of the stator and rotor rings against each other. The energy transmitted from such generators into the medium is generally quite small. For example, continuous primary transesterification with a multiple-stage high-shear mixer with a 40 gallon-per-minute (gpm) capacity would require 30 horsepower (hp); and continuous secondary transesterification with a single-stage high-shear mixer for 40 gpm capacity would require 20 hp.

High-shear mixing is a rapidly emerging processing technique in renewable energy to improve yield, product quality and reduce processing times.

Michael Janssen is regional sales manager/renewable energy specialist with IKA Works Inc. Reach him at [email protected].
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