Helmut Brod

Residence time optimised choice of tube diameters and slit heights in distribution systems for non-Newtonian liquids

Abstract In order to minimise thermal degradation effects such as yellowing and losses in molecular weight, distribution systems for polymer melts and solutions should be designed for residence times as small as possible for a given value of the total pressure loss. Tube and slit flow distributors that can be considered as mainly one-dimensional systems are analysed with respect to an optimal ratio of technical expenditure (pressure loss) and gain in polymer quality (reduction in residence time, i.e. in thermal degradation). Assuming steady, fully developed, laminar flow and excluding changes in the rheology of the liquid with residence time, this leads to mathematical optimisation problems which may be solved by the methods of the calculus of variations. The definition of average residence time turns out to be ambiguous for systems with more than one inlet or one outlet. The problem may be defined in two alternative ways, using different measures of residence time. One of these time measures, called “travel time”, turns out to be superior as a criterion for the optimisation of distributors for polymeric liquids. Based on this criterion, a general design rule for optimising slit and tube distributors is derived. This rule states that, in order to minimise the travel times within a system, it is sufficient to choose the diameter of the tubes or the slit width in such a way that the local apparent wall shear rate is uniform throughout the whole system. This rule has a broad range of applicability. It holds for arbitrary non-linear flow curves of the liquid under consideration and for arbitrary total flow rates. In addition, the choice of a uniform wall shear rate and, hence, a uniform wall shear stress safeguards the highest level of protection against wall fouling throughout the system at the lowest possible cost (pressure loss).

Improving Cell Culture Bioreactor Performance for Sensitive Cell Lines by Dynamic Membrane Aeration (DMA)

Although the importance of animal cell culture for the industrial (large scale) production of pharmaceutical products is continuously increasing, the sensibility of the cells towards their cultivation environment is still a challenging issue. In comparison to microbial cultures, cell cultures which are not protected by a cell wall are much more sensitive to shear stress and foam formation. Reactor design as well as the selection of “robust” cell lines is particularly important for these circumstances. Nevertheless, even “sensitive” cell lines are selected for certain pharmaceutical processes due to various reasons. These sensitive cell lines have even higher requirements regarding their cultivation environment. Important characteristics for the corresponding reactor design are a high (volumetric) gas mass transfer coefficient, low volumetric power input, low shear stress, low susceptibility to bio-fouling, the ability to cultivate sticky cells and sufficient mixing properties. Membrane aeration has been a long-known possibility to meet some of these requirements, but has not often been applied in recent years. The reasons lie mainly in low gas mass transfer rates, a limited installable volume-specific membrane surface area, restrictions in scalability and problems with membrane fouling. The dynamic membrane aeration bioreactor aeration is a simple concept for bubble-free oxygen supply of such sensitive cultures. It overcomes limitations and draw-backs of previous systems.