R.M.J. van Damme

The use of the dusty-gas model for the description of mass transport with chemical reaction in porous media

In the present study, mass transport accompanied by chemical reactions in porous media is studied according to the Fick model and the dusty-gas model. For mass transport accompanied by a chemical reaction in catalyst structures showing a plane, line, or point of symmetry, the approximate analytical concept of an effectiveness factor, accounting for intraparticle diffusion, was also evaluated. For a variety of reaction schemes and kinetic rate equations, a comparison was made between the results of the numerical models (Fick and dusty-gas) and the effectiveness-factor concept. From the results it was concluded that pressure in porous catalyst with a plane, line, or point of symmetry did not affect the fluxes seriously, and, therefore, the pressure-driven flow can be omitted from the flux expression without significant loss of accuracy. Furthermore, both for single and multiple reactions, the Fick model is satisfactorily accurate to estimate the transport rate in all cases, and the results deviate only slightly from the dusty-gas model. It should be noted that this latter model requires substantially more computational time. For catalytic membranes, however, transport of inert components as well as large trans-membrane pressure differences may be present, which affect the transport of the reactants and products. The calculations showed that, in contrast to the above-mentioned structures, in this case the dusty-gas model has to be used to describe the transport.

A catalytically active membrane reactor for fast, exothermic, heterogeneously catalysed reactions

A membrane reactor with separated feed of reactants is demonstrated as a promising contractor type when dealing with heterogenously catalysed, very fast and exothermic gas phase reactions. Due to the separation of reactants a good control of the system is obtained, because process variables can be varied independently from each other. Transport of reactants is the rate governing process and because this is only slightly temperature dependent a thermal runaway will not occur. When dealing with e.g. combustion process no explosive mixtures will build up and safety is increased. Based on the dusty-gas model, the concentration profiles of components inside the membrane can be calculated together with the fluxes. However this is a calculation time consuming process and not necessary in all cases. In absence of a pressure drop and no slip of reactants to the opposite side a linearisation is possible leading to a simplified expression for the interfacial flux of a reactant and a criterion to evaluate the possibility of slip of reactants. Using the oxidation of carbon monoxide catalysed by platinum as a model reaction this approximation was experimentally verified by comparison of measured fluxes with the calculated results. Apart from flux measurements exploratory overall conversion measurements were carried out with the membrane reactor module in order to demonstrate its operation performance. From these studies it was concluded that conversion levels up to 90% carbon monoxide could easily be achieved.

Coupling losses in superconducting, torus-shaped wires due to applied magnetic field changes

The stationary electric field, current pattern and coupling losses in a multfilamentary, superconducting, twisted, torus-shaped wire are calculated for a torus placed in a homogeneous magnetic field increasing in time at a constant rate and parallel to the torus plane. The radius of the wire is considered to be small compared to the mean radius of the torus. An important parameter for the problem is the ratio between the twist length of the superconducting filaments and the mean radius of the torus. In the configuration considered this parameter is small. The coupling losses are approximately inversely proportional to the square of this ratio. Furthermore, for the wire to have unsaturated parts, the analysis shows that the rate of change of the magnetic field must decrease when this ratio increases.

APPLICATIONS OF A NON-PERMSELECTIVE, CATALYTICALLY ACTIVE MEMBRANE, A MODEL STUDY

A theoretical investigation has been presented for applications and features of a non-permselective, catalytic membrane reactor with separated feed of reactors. Transmembrane fluxes were calculated from the dusty gas model as a function of a great number of parameters and operation conditions. This study shows that the non-permselective, catalytic membrane reactor with separated feed of reactants (CMRSR) has attractive features to use this reactor in fast and highly exothermic reactions and selectivity improvement in multiple reactions. When the CMRSR is operated in the transport controlled regime, the process is easy to control and even possesses some self-controllability. Due to the transport conversion, thermal runaway cannot occur which allows operation with concentrated feed of reactants. Furthermore, a transmembrane pressure difference increases both the fluxes and the selectivity, because the reaction products are preferentially directed towards one side of the membrane. The simultaneous increase of both selectivity and fluxes is a remarkable feature of a CMRSR which is in contrast with conventional reactors.

Transient Response of a Composite Superconducting Wire in Spatially Periodical Longitudinal A.C. Magnetic Fields

In this paper analytical results are presented concerning the transient response of a composite superconducting wire in spatially periodical longitudinal ac magnetic fields. Locally solutions are positively, negatively or unsaturated in a region and the positions of the interfaces between different regions are calculated. One dominant time constant has to be included in the analysis due to the twist. Other time constants can be neglected, because the periodicity length of the field is assumed to be much larger than the radius of the wire. We focus on the influence of the dominant time constant on the interfaces.