Scale-up of Continuous Mixers

Scale-Up of Continuous Mixers Although scaling up on the basis oFconstant power per unit Feed rate [kWh/lcg or (hp-h)/lb] is iisii-allv a good First estimate, several other Factors rnav have to be considered, As the equipment scale is increased, geometric similarity being... 

Scale-up of Continuous Mixers While geometric similarity may be practical for most batch mixers, changes in the length-to-diameter ratio or other geometry may be necessary with continuous mixers. The most common problem is heat generation by friction and heat removal by surface transfer. 

The throughput rate of intermeshing twin-screw extruders (Fig. 18-56) and the Farrel continuous mixer (Fig. 18-51) is scaled up with diameter to about the 2.6 power. 

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. 

After the molecular weight and composition of the block polymer were fixed at appropriate levels, mechanical blending was scaled-up to both larger and continuous dispersion mixers. An acceptable and reproducible product was made at all levels up to the highest output tried, e.g., 1000 lbs/hr. 

Crude oil and brine pumps may be centrifugal or positive displacement, but must be capable of providing steady flow to the mixing device because emulsion properties are highly dependent on the resulting crude-oil-brine ratio. Surfactant may be dissolved in the brine phase on a batch or continuous basis. Static mixers provide a simple method for the preparation step because they require no moving parts, are easy to scale up, and provide an mixing intensity that is suited to preparation of transport emulsions. 

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)... 

As the result of the departure from geometric similarity, the throughput rate of single-screw extruders scales up with the diameter to 2.0 to 2.5 power, instead of the diameter cubed, at constant length-to-diameter and screw speed. The throughput rates of twin-screw extruders (Fig. 18-57) and the Farrel continuous mixer (Fig. 18-58) are scaled up with the diameter to about the 2.6 power. 

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. 

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