Scale-up and Testing of Mixers

In this section we outline the commonly accepted scale-up criteria used in industry for agitated mixers and silo blenders. Scale-up of tumbling mixers was addressed in Section 15-5. The complexity of interaction between physical properties of solids, mixer configuration and velocity, and stress profiles within a mixer makes it difficult to formulate generalized scale-up criteria. Various experimental investigations into scale-up, however, do provide useful guidance for scale-up. No concurrence on acceptable approach has been reached, and various manufacturers tend to follow their experience. 

A general axiom for scale-up is Conunit your blunders on a small scale, and make your profits on a large scale (Zlokamik, 1984). Although it is easy to conduct experiments on a small scale, one must also consider the following questions  

What geometric, dynamic, and kinematic parameter ratios need to be kept constant to ensure the validity of data on a large scale  

Are the material properties (e.g., particle size distribution, temperature, and moisture) comparable to those in a large scale process  

Are there other processes (heating, cooling, granulation) that require scale-up along with the mixing process  

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SCALE-UP AND TESTING OF MIXERS 971 Criterion 2 Keep Froude numbers constant ... 

For reliable scale-up and extrapolation of test data, it important to maintain geometric, kinematic, and dynantic similarity between the test mixer and the fiill scale mixer. A batch mixer should have access for sampling within the mixer at various mixing times. Batch ntixers require reliable dosing measurement, which can be accomplished by weighing the components individually. 

Scale-up techniques for using the results of pilot plant or bench scale test w ork to establish the equivalent process results for a commercial or large scale plant mixing system design require careful specialized considerations and usually are best handled by the mixer manufacturer s specialist. The methods to accomplish scale-up will vary considerably, depending on whether the actual operation is one of blending, chemical reaction tvith product concentrations, gas dispersions, heat transfer, solids suspensions, or others. 

The initial bench-scale experimental investigations into solvent extraction processes are conducted with small apparatus, such as separating funnels. Following the successful completion of these tests, when the best reagent and other conditions for the system have been established, small-scale continuous operations are run, such as in a small mixer-settler unit. The data so obtained are used to determine scale-up factors for pilot plant or plant design and operation (see Chapters 7 and 8). 

Laboratory Extractors. Pilot-Scale Testing, and Scale-Up. Several laboratory units arc useful in analysis, process control, and process studies. The AKUFVE contactor incorporates a separate mixer and centrifugal separator. It is an efficient instrument for rapid and accurate measurement of partition coefficients, as well as for obtaining reaction kinetic data. Miniature mixer-settler assemblies set up as continuous, bench-scale, multistage, countercurrent, liquid-liquid contactors are particularly useful Tor the preliminary laboratory work associated with flow-sheet development and optimization because these give a known number of theoretical stages. 

In 1968, an electrolytic reduction process was proposed by A. Schneider and A. L. Ayers (6) to circumvent the above disadvantages. A research program was carried out in the Allied Chemical Corporation s laboratories during the years 1968 to 1972 to develop the process and equipment. The work resulted in the development of the Electropulse Column ( 7) for the continuous (differential) electrolytic uranium-plutonium partition process, which was later scaled up, fabricated, and installed in the Allied-General Nuclear Services reprocessing plant at Barnwell, South Carolina. About the same time, a stagewise electrolytic uranium-plutonium partition process was tested on a mini mixer-settler unit in Germany. (8)... 

Therefore, nearly all manufacturers of agglomeration equipment maintain rather elaborate laboratories, often with large scale equipment to avoid scale-up problems. These test facilities must also include peripheral equipment such as mixers, driers, heaters, crushers, screens, etc. For cost reasons, in most cases no continuous operation of an entire production line is possible. To simulate expected plant characteristics, a certain amount of fines may be recycled to the agglomeration unit. The amount and typical size consist of the recycle stream will be determined and, after simulated production, will be mixed with the fresh feed to evaluate the influence of this feed component on projected plant performance. 

While the pilot plant approach during project development must be investigated and decided upon on an individual basis, testing of equipment is always necessary. For that reason, essentially all manufacturers and/or vendors maintain sometimes rather elaborate facilities (Section 9.1 and [B.48, B.97]), which normally feature machines of different sizes, including, in some cases, large scale equipment, to avoid scale-up problems. These test facilities must also include some peripheral equipment, such as mixers, heaters or coolers, conveyors, crushers, dryers, screens, etc., although the variations that are available in these special areas from outside sources can not be offered. Therefore, additional tests for the evaluation and selection of the best peripheral equipment are often necessary at different facilities. 

The laboratory machines are often used for the evaluation of compounds on a small scale, prior to production. The test requires a small amount of materials, short time, little efforts and operational expense. The scale-up of the test results to production size is usually done through the unit work concept. The mixer makes it possible to assess the effects of changes in temperature, torque, and shear characteristics. 

Investment costs for industrial mixer-setders are high. Extraction columns (Figure 2.3.4-10), and in some cases also special constructions, are much cheaper. However, in this case scale-up is problematic. For example, in large equipment widespread back-mixing or poor initial distribution of the disperse phase can occur, although they are not observed in the small dimensions of the test apparatus. Such effects can lead to dramatic drops in separation performance. 

A viscous material (power-law rheology) is to be processed in a mixing vessel under laminar conditions. A sample of the material is tested using a laboratory scale mixer. If the mixing time is to be the same in the small and large-scale equipments, estimate the torque ratio for a scale-up factor of 5 for the range 1 > n > 0.2. 

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