Microstructured mass transfer resistance

In microfluidic-based systems, material is transported within microstructures (of typical dimensions of 10-500 pm) where separations, reactions, and other processes occur. Focus has been on the realization of the traditional separation techniques (electrophoresis, chromatography, isoelectric focusing, etc.) and reactions in the microchip format. The principles of separation, as in the conventional formats, are based on differences in mass and charge (thus mobility) and partitioning between phases. However, advantages associated with the small dimensions provide superior performance. For example, the higher surface to volume ratio arising from the smaller dimensions results in lower heat and mass transfer resistances and thus an improved performance. 

Multiphase microstructured devices can potentially be used to dimmish the limitations of conventional reactors. They generally take advantage of their large interfacial area reducing the mass transfer resistances. 

Regarding the solvent used to prepare the catalyst ink, its properties in catalyst ink should be mentioned as it also plays an important role in determining the microstructure and cataljAic activity of the CL. When ionomer such as Nafion solution is mixed with solvent, the mixture may become a solution, a colloid, or a precipitate due to the different dielectric constants of the solvent. When the dielectric constant is more than 10, a solution is formed between three and 10, a colloidal solution is formed and less than 3, precipitation occurs.If the mixture is a solution (i.e., the solution method ), excessive ionomer may cover the carbon surface, resulting in decreased Pt utilization. However, when the mixture is a colloid (the colloidal method ), ionomer colloids adsorb on the catalyst powder and the size of the catalyst powder agglomerates increases, leading to an increased porosity of the CL. In this case, the mass transfer resistance could be diminished because of the continuous network of ionomers throughout the CL, which then improves the proton transport from the catalyst to the membrane. ... 

To eliminate mass transfer resistances in practice, the characteristic transfer time should be roughly one order of magnitude smaller than the characteristic reaction time. As the mass andheat transfer performance ina microstructured reactor (MSR) is up to two orders of magnitude higher than in conventional tubular reactors, the reactor performance can be considerably increased, leading to the desired intensification of the process. In addition, consecutive reactions can be efficiently suppressed due to strict control of the residence time and a narrow residence time distribution (RTD). Therefore, fast reactions carried out in a M SR show higher product selectivity and yield. 

Keywords polymerization kinetics, polymerization reactors, mathematical modelling, molecular weight distribution (MWD), chemical composition distribution (CCD), Ziegler-Natta catalysts, metallocenes, microstructure, isotacticity distribution, mass transfer resistances, heat transfer resistances, effects of multiple site types. 

An electrode model is especially advantageous if it can be used to relate the kinetic and mass transfer resistance to electrode geometry and microstructure for instance, to thickness, porosity, pore or particle size, contact areas of phases, and/or grain size of electrode and electrolyte materials. A well-tested and validated electrode model, therefore, may serve to assist in the design of optimised electrode structures or electrode/electrolyte interfaces to minimise polarisation loss. 

In contrast, the finger-like microvoids or channels inside microstructured ceramic hollow-fiber membranes significantly reduce mass transfer resistance, and the thin sponge-like skin layer can fimction as a separation layer. Thus, a single-step viscous... 

Figure 8.8 Mass transfer in dispersed phase microstructured reactors where gas solute diffuses through liquid toward solid surface, (a) Schematic representation, (b) Resistance model.

Various parameters must be considered when selecting a reactor for multiphase reactions, such as the number of phases involved, the differences in the physical properties of the participating phases, the post-reaction separation, the inherent reaction nature (stoichiometry of reactants, intrinsic reaction rate, isothermal/ adiabatic conditions, etc.), the residence time required and the mass and heat transfer characteristics of the reactor For a given reaction system, the first four aspects are usually controlled to only a limited extent, if at aH, while the remainder serve as design variables to optimize reactor performance. High rates of heat and mass transfer improve effective rates and selectivities and the elimination of transport resistances, in particular for the rapid catalytic reactions, enables the reaction to achieve its chemical potential in the optimal temperature and concentration window. Transport processes can be ameliorated by greater heat exchange or interfadal surface areas and short diffusion paths. These are easily attained in microstructured reactors. 

Figure 1 Effect of multiple site types and mass and heat transfer resistances on the microstructure of polypropylene made with heterogeneous Ziegler-Natta and metallocene catalysts. The overall MWD and CCD are assumed to result from the superposition of individual MWDs and CCDs for three site t)rpes (T = temperature, M = number average molecular weight, = hydrogen, CjH = propylene, C2H4 = ethylene, Fj = molar fraction of propylene in copolymer, /(F,) == copolymer composition distribution, r = chain length, wix) = weight chain length distribution).

Simplified models that do not make a priori assumptions about one or more dominant resistances are often of the 1-D macrohomogeneous type. The 1-D assumption is similar to that in mass transfer-based models. The assumption of macrohomogeneity, based on work by Newman and Tobias [50], has proven useful in battery and fuel cell electrode modelling. It implies that the microstructure of the electrode is homogeneous at the level of the continuum equations governing mass transfer, heat transfer, and current conduction in the electrode (Eqs. (l)-(7) and (33)-(37)). This type of model can exploit solutions available in chemical reaction engineering practice and has been elaborated by several researchers in that field [51-55],... 

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