Dense process performance control

When Dense or Microporous Materials Control the Overall Process Performance... 

For membrane operations in which dense or microporous materials control the overall process performance there is no doubt that process intensification will follow directly from improvement of material properties. With the development of new materials having properties controlled at the nanoscale level, operations of this kind seem promised a really bright future. 

Membrane reactors can be considered passive or active according to whether the membrane plays the role of a simple physical barrier that retains the free enzyme molecules solubilized in the aqueous phase, or it acts as an immobilization matrix binding physically or chemically the enzyme molecules. Polymer- and ceramic-based micro- and ultrafiltration membranes are used, and particular attention has to be paid to the chemical compatibility between the solvent and the polymeric membranes. Careful, fine control of the transmembrane pressure during operation is also required in order to avoid phase breakthrough, a task that may sometimes prove difficult to perform, particularly when surface active materials are present or formed during biotransformahon. Sihcone-based dense-phase membranes have also been evaluated in whole-cell processes [55, 56], but... 

Plasma spraying is a consolidation process for powders with the additional capability of a composition control of the spray formed structures. The paper reports on the first steps to adapt this method to the production of functionally graded thermoelectric materials with a locally maximized figure of merit. Iron disilicide (FeSi2) was used to test the performance of the technique on thermoelectric material. It was found that plasma spray forming is applicable to produce dense materials with thermoelectric properties comparable to hot pressed ones. Problems were however found with the thermal stability of the microstructure. 

SOFC anodes is typically a complex inter-networks of ionically and electronically conducting phases, and gas-filled porosity. Control of the composition and micro-structure is critical for the activity of electrodes [6]. Percolating networks of three-phase boundaries formed by the electronic phase, ionic phase, and the gas-phase are important for high electrochemical performance of the cell. A three-dimensional reconstruction of a typical state of the art Ni/YSZ anode and its three-phase boundaries reproduced from [7] is shown in Fig. 1.2. There are numerous techniques by which the anodes can be fabricated [8,9]. In all cases the NiO-YSZ active layer as fired is a dense material, and most of the porosity results during the reduction process [7]. Zhu et. al [10] reported that a continuous porosity of more than 30% is required to facilitate the transport of reactants and products to and away from the three-phase boundary (TPB). 

Relationship of reduction process and catalytic activity. Prior to reduction, fused iron catalyst is a dense solid and without catalytic activity. The activity of the fused iron catalyst is not only related with the chemical components and preparation method, but also dependent upon the reduction process because all physical properties such as the surface area, porous structure, pore size and distribution, specific volume of pore, especially, size and formation of a-Fe crystallite etc. are produced during the reduction process. Different reduction processes produce different physical properties, and the surface area and porous structure are also different. The high performance catalysts can only be obtained when the reduction process are carefully controlled. Therefore, the reduction of the catalysts is the last step of the catalysts preparation, and also a key step. 

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