Microscale processes

The subject of this chapter is the relationship between macroscale observations and the underlying microscale processes in shock compression. Since the greater part of our current experimental knowledge of the shock compression process involves macroscale observations, we try to infer microscale phenomena from these data. A much more satisfactory approach is the direct real-time observation of microscale processes themselves. This is difficult to do in most cases, so we must still rely on a combination of macroscale measurement, microscale theory, and whatever direct observations of microscale processes that can be made. 

The shear rate between liquid layers moving at the average velocity is an important factor in macroscale processes involving large blobs. The velocity fluctuations are responsible for microscale processes of homogenization at the molecular level. The data for turbine agitators show that the mean velocity fluctuation, defined as... 

Nowadays, CFD research at academia is heavily engaged in attempts to include microscale transport phenomena and microscale processes in the dedicated codes under development with a view to reproduce such divergent processes as... 

In addition, the turbulent fluctuations set up a microscale type of shear rate. Microscale mixing tends to affect particles that are less than 100 /xm in size. The scaleup rules are quite different for macroscale controlled process in comparison to microscale. For example, in microscale processes, the major variables are the power per unit volume dissipated in various points in the vessel and the total average power per unit volume. In macroscale mixing, the energy level is important, as well as the geometry and design of the impeller blades and the way that they set up macroscale shear rates in the tank. 

On the other hand, the turbulent velocity fluctuations RMS v affect microscale processes, again variously estimated at 100 to 200 microns or less, on down to possibly the mixing length or possibly on down to the scale of molecular reaction. 

Charcoal kiln processes can be divided into two groups, referred to here as microscale processes and macroscale processes. The former group encompasses those processes occurring close to or within individual wood particles. The latter group includes larger scale processes such as the convective transport of heat through the wood stacked in the kiln. 

Microscale equipment has high surface-to-volume ratios, which facilitates designs with high heat and mass transfer rates. Hessel, Hardt and Lowe have written a complete review of the current state of microscale processing equipment... 

Doig, S. D., Pickering, S. C. R., Lye, G. J., Woodley, J. W., The use of microscale processing technologies for quantifr-cation of biocatalytic Baeyer-Villiger oxidation kinetics, Biotechnol. Bioeng., 80(1), 2002,42-49. 

BALES SZECSODY Microscale Processes in Porous Media... 

Detrital plant tissue is detached from original plant and deposited on the soil surface. Various macro- and microscale processes (as discussed in the latter part of this chapter) are involved in the breakdown of this material. 

Plant detritus (either attached or detached from the plant) undergoes physical fragmentation through the action of waves and currents, UV exposure, and through grazing activities of macro invertebrates. Fragmentation of plant detritus into small fractions (<1 mm) results in increased surface areas and accelerated microscale processes (such as enzymatic hydrolysis and catabolic activities). 

The microscale processes demonstrated in Fig. 3 are intrinsic to SSF due to the particulate nature of the substrate. They occur in all bioreactors and relatively little can be done to influence them in the way the bioreactor is designed and operated since they occur at the surface and inside the individual substrate particles. The most that can be achieved through bioreactor design and operational strategies is to promote exchange between the particle and air phases and to ensure that the transport processes within the air phase are not limiting. 

Despite our very hmited ability to influence these processes in the way we operate the bioreactor, it is still essential to understand their influence on the system. Understanding how and when microscale processes control process performance can prevent unfruitful attempts to improve performance by manipulating the operational variables of the bioreactor. Such understanding might point to more useful strategies. For example, for a process controlled by intraparticle mass transfer, it might be possible to disrupt barriers to diffusion within the substrate particle, such as plant cell walls. Independently of these reasons, characterization of at least some of the microscale phenomena is necessary for the construction of appropriate expressions to include in macroscale material and energy balances. 

Although the various microscale processes are all interrelated, as shown in the diagram, they will be discussed one by one. 

The realization of the comparison between macroscale semi-continuous batch and a continuous microscale process on the laboratory scale was carried out for several reasons. First, it was of marked interest whether ecological improvements can be expected for the chosen model reaction performed in microstructured devices. Such improvements were the premise for the transfer of this model reaction to the... 

Interfacial surface structure and interaction of materials at the nanoscale have attracted much attention in the field of nanotechnology (Drexler 1992). Microbiological systems have been investigated as a microscale process (Zhao et al. 1998) however, recent studies showing the unique interaction of biological systems with entirely synthetie molecular assemblies have prompted... 

The second milestone in chemical engineering came in 1960 with the publication of Transport Phenomena, by Bird et al. [2]. Their new approach emphasized the microscale processes and the analogy among mass, heat, and momentum transfer in different processes. 

In mioofluidic reactors, the high-pressure, high-temperature requirements of many industrial reactions are combined with the need to carry out the reactions on a small scale to obtain enhanced seleclivities. The advances in microscale processing have made it possible to design and construct systems with micron-sized diameters. 

Apparently, it was found apprehensive to know a thermodynamic system in general for which we routinely need some representation about certain in-depth make-up (microscale processes), which, however, may be dependent on the model applied. We may recall Gibbs citing ...in spite of certain... 

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