Scale down, mixing

When reacting two phases that are not very soluble in each other, for example when carrying out nucleophilic substitution reactions, phase transfer catalysts should be considered when scaling down from equipment with poor mixing characteristic, rather than buying new equipment. 

Another Lagrangian-based description of micromixing is provided by multienvironment models. In these models, the well macromixed reactor is broken up into sub-grid-scale environments with uniform concentrations. A four-environment model is shown in Fig. 5.16. In this model, environment 1 contains unmixed fluid from feed stream 1 environments 2 and 3 contain partially mixed fluid and environment 4 contains unmixed fluid from feed stream 2. The user must specify the relative volume of each environment (possibly as a function of age), and the exchange rates between environments. While some qualitative arguments have been put forward to fit these parameters based on fluid dynamics and/or flow visualization, one has little confidence in the general applicability of these rules when applied to scale up or scale down, or to complex reactor geometries. 

Fabrication issues can arise when thinning the separator material or in trying to ensure high-quality, preferably pinhole-free coverage of dimensionally scaled-down anodes and cathodes. One strategy that can be used to avoid fabricating a separator at all, at least in electrolyzers and fuel cells, takes advantage of decades of work in compact mixed... 

In mixing applications, scale-up is indeed concerned with increasing the linear dimensions from the laboratory to the plant size. On the other hand, processes exist (e.g., tableting) where the term scale-up simply means enlarging the output by increasing the speed. To complete the picture, one should point out special procedures (especially in biotechnology) where an increase of the scale is counterproductive and scale-down is required to improve the quality of the product. 

If Small scale experiments are to be used for evaluation purposes, then they must be designed to have blend time, pumping capacity and shear rate levels relatively similar to full scale installation. This normally means a non-geometric model of scale-down to control various mixing parameters. 

Research and Development Laboratories (ERDL) in Baton Rouge, La. This unit was designed to be similar in performance to the stirred alkylation units currently in commercial use. Although it was not possible to maintain exact mixing similarity, this unit maintained similitude in the more important scale-down parameters. 

Scale-up/Scale-down Two aspects of scale-up frequently arise. One is building a model based on pilot-plant studies that develop an understanding of the process variables for an existing full-scale mixing installation. The other is taking a new process and studying it in the pilot plant in such a way that pertinent scale-up variables are worked out for a new mixing installation. 

Because every broadband Hartmann-Hahn mixing sequence has only a finite bandwidth, it can, in principle, be turned into a band-selective Hartmann-Hahn mixing sequence by scaling down the rf amplitude of the sequence. Then coherence transfer is restricted to the scaled active bandwidth and coherence transfer to spins that are well outside of this... 

Gilbert and Martin have described an excellent kinetics experiment which illustrates solvolysis of 2-chloro-2-methylbutane via an SnI mechanism (10). The reaction is carried out in a green solvent mixture of water and 2-propanol. In addition, we have scaled down to the use of only 118 microliters of 2-chloro-2-methylbutane per student, and carry out our titrations using reusable plastic syringes instead of burets (77). This lab exercise provides experimental support for the proposed SnI mechanism and illustrates the effect of solvent polarity (increasing percentages of water in the solvent mix) on reaction rate. 

When reactions are fast relative to the mixing rate, not only are the apparent reaction rates affected but the whole time and temperature history of the reaction mechanism is also affected, yielding different selec-tivities and yields, depending on the intensity of the mixing. This often leads to a scale-up/scale-down problem, where yields of the desirable products in a plant-scale reactor are not as good as those in a small-scale reactor in the laboratory or the pilot plant. If the yield drops from the pilot-scale to the plant-scale reactor when all other important variables (temperature, pressure, and composition) have been held constant, then there is a mixing problem. Fast... 

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