A fluid-dynamically based model of bubble column reactors

A heterogeneous fluid dynamic model has been developed to describe the complex flow structure of two-phase in bubble columns. The equation of continuity and momentum balances form the basis of the model. Coupling of the two phases occurs via an interaction force, deduced by a force balance around a single rising bubble. Multiphase flow mixing processes are taken into consideration by introducing turbulent viscosities of the two phases involved. The Simulation program was extended to reactive system, taking into account the mass balances of a second order gas-liquid chemical reaction as well as the different absorption/reaction regimes. The gas phase concentration profiles show pronounced axial and radial dependences, while the liquid phase can be regarded as a CSTR with respect to the liquid component. With reference to the gaseous component, which is being absorbed in the liquid phase, the degree of back mixing does not show CSTR behaviour as the influence of different absorption conditions in different axial and radial reactor positions is superposed on that of turbulent motion of the liquid carrier of the dissolved gaseous component.     

Superconductor Stability Revisited: Impacts from Coupled Conductive and Thermal Radiative Transfer in the Solid

Standard stability calculations following the Stekly, adiabatic or dynamic stability models apply purely solid thermal conduction mechanism, derive predictions under (quasi-) stationary and adiabatic conditions and assume ideal (mostly centrosymmetric) location of disturbances. Instead, the present paper takes into account also thermal radiative heat transfer in the superconducting solid, pool boiling, and considers the impact of random location and intensity of disturbances on the stability problem. The analysis is based on interplay between Monte Carlo radiative transfer calculations and a rigorous Finite Element method to calculate the resulting transient temperature field and stability functions. The combined Monte Carlo/Finite Element method is applied to 1G filament and 2G thin-film-coated high temperature superconductors. Results are strongly different from solutions achieved with standard, solely solid conduction thermal transport. It is not realistic, even in thin films, to assume uniform conductor temperature under transient disturbances. This may have significant consequences for design and simulation of performance of superconducting fault current limiters. There are doubts whether superconducting fault current limiters under any operation conditions could work in either pure flux flow or Ohmic resistive states.     

Waste disposal and waste avoidance

Waste disposal and waste avoidance are examined from two different perspectives, from the point of view of the waste 'producing' company on the one hand, and from the point of view of the waste disposing company on the other hand. The idea of the basic model lies in the 'turning upside down' of the lot-sizing approach to inventory control theory. We derive a corresponding formula for disposal that shows that the number of waste disposals depends positively on the waste accumulation rate and negatively on the fixed costs for a pick-up. Moreover, we formally work out at which waste avoidance costs a company will enlist the services of a disposal company. Specifically, disposal tends to be preferred to avoidance when waste-avoidance is relatively expensive and three cost rates to be specified are relatively low. Finally, we take the perspective of a monopolistic disposal company that has to decide on the optimal pricing policy. In our model, we determine that the price for a pick-up should be chosen just to cover costs. The disposal company can earn a profit by the suitable choice of the price per unit waste to be disposed of.     

Influence of growth conditions on biofilm development and mass transfer at the bulk/biofilm interface

In a long-term study on heterotrophic biofilms in tube reactors, this investigation focused on mass transfer at the bulk/biofilm interface, biofilm density and substrate conversion rates. Several biofilms were cultivated under different substrate and hydrodynamic conditions. Oxygen concentration profiles were measured with microelectrodes in the biofilm and in the boundary layer directly in the biofilm tube reactors. The thickness of the concentration boundary layer was found to depend on the surface structure of the biofilm. The hydrodynamic conditions and the substrate load during the growth phase of the biofilm in biofilm systems are two key parameters that influence the biofilm growth, particularly the structure, density and thickness. The measured substrate conversion rates, biofilm densities and the boundary layer thickness were used to formulate an equation for the mass transfer in biofilm tube reactors.