Arno Kwade

Wet comminution in stirred media mills — research and its practical application

Abstract The importance of stirred media mills in various industries and in research is steadily increasing. The comminution process in these mills is determined mainly by the number of stress events in the grinding chamber and by the intensity acting at these stress events: for a certain stress intensity a certain relation between the product fineness and the number of stress events exists. Since the product of stress number and stress intensity is a measure for the specific energy, for each stress intensity also a certain relation between product fineness and specific energy exists. At similar stress intensities the influence of stirrer and grinding chamber geometry is small. Moreover, the influence of residence time distribution and mill geometry on the product particle size distribution, wear and process models are discussed.

Breaking characteristics of different materials and their effect on stress intensity and stress number in stirred media mills

The comminution process in stirred media mills is determined mainly by the number of stress events in the grinding chamber and by the intensity acting at these stress events. For a certain stress intensity, a certain relation between the product fineness and the number of stress events exists. Since the product of stress number and stress intensity is a measure for the specific energy, for each stress intensity, a certain relation between product fineness and specific energy also exists. The effect of stress intensity, stress number and specific energy on the product fineness is determined by the breakage characteristic of the material. With respect to the breakage characteristic, two main groups can be distinguished: firstly, deagglomeration and disintegration and secondly, grinding of crystalline materials. Among others, the breakage characteristics determine how strongly the specific energy consumption is affected by the stress intensity.

Characterization and control of fungal morphology for improved production performance in biotechnology.

Filamentous fungi have been widely applied in industrial biotechnology for many decades. In submerged culture processes, they typically exhibit a complex morphological life cycle that is related to production performance--a link that is of high interest for process optimization. The fungal forms can vary from dense spherical pellets to viscous mycelia. The resulting morphology has been shown to be influenced strongly by process parameters, including power input through stirring and aeration, mass transfer characteristics, pH value, osmolality and the presence of solid micro-particles. The surface properties of fungal spores and hyphae also play a role. Due to their high industrial relevance, the past years have seen a substantial development of tools and techniques to characterize the growth of fungi and obtain quantitative estimates on their morphological properties. Based on the novel insights available from such studies, more recent studies have been aimed at the precise control of morphology, i.e., morphology engineering, to produce superior bio-processes with filamentous fungi.

Motion and stress intensity of grinding beads in a stirred media mill. Part 2: Stress intensity and its effect on comminution

Experimental investigations concerning the comminution of limestone in a stirred media mill have been carried out. The results show that the circumferential velocity of the stirrer discs as well as the density and the size of the grinding beads affect the specific energy consumption necessary to achieve the required product fineness. The influence of these three operating parameters on the comminution result can be fully described by the stress intensity of the grinding beads. Therefore, specific energy consumption and stress intensity of the grinding beads are the comprehensive influencing variables on the comminution of limestone in stirred media mills. It is expected that this relationship is also valid for the comminution of other materials in stirred media mills. For a fixed specific energy input, an optimum stress intensity exists, for which the finest product is achieved. With increasing specific energy input, and therefore increasing product fineness, the optimum stress intensity decreases. Since the specific energy is proportional to the product of stress intensity and stress frequency, the comminution result can also be correlated to the stress frequency and the stress intensity. With increasing stress intensity, the stress frequency required for a certain product fineness decreases.

Determination of the most important grinding mechanism in stirred media mills by calculating stress intensity and stress number

Abstract Based on the motion of the grinding media in stirred media mills with disc stirrer different grinding mechanisms are possible: product particles can be stressed by grinding media which are accelerated from the stirrer shaft towards the grinding chamber wall (A), which are pressed against the chamber wall because of the centrifugal acceleration (B) and which move in tangential direction with high velocities and collide with grinding media with lower velocities (C). The importance of the three different grinding mechanisms can be determined by evaluating the respective stress intensity and the respective number of stress events per unit time. The theoretical investigations showed, that the product particles are broken mainly by stress mechanism C. This finding was confirmed by experimental investigations concerning the comminution of limestone in a stirred media mill.

Motion and stress intensity of grinding beads in a stirred media mill. Part 1: Energy density distribution and motion of single grinding beads

Abstract Numerical calculations concerning the flow field, the distribution of the specific energy and the motion of single grinding beads in the grinding chamber of a stirred media mill have been carried out. The calculations are based on steady-state laminar stirring of a Newtonian fluid without grinding media. The flow field of the stirred fluid generates a characteristic distribution of the specific energy. Two zones characterized by a high energy density exist. In these zones the local specific energy is larger than the mean specific energy which is obtained by dividing the total amount of energy dissipated in the grinding chamber by the net volume of the grinding chamber. One zone extends around the stirrer disc whereas the other is located at the grinding chamber wall. The vilume of these two zones is only about 10% of the net grinding chamber volume. Approximately 90% of the entire energy input is dissipated there. Single grinding beads that are exposed to a previously determined flow pattern tend to follow an almost stationary individual trajectory in the grinding chamber. The position of the trajectory depends on the ratio of bead-to-fluid density, the ratio of bead-to-disc radius and the Reynolds number which describes the operating conditions of the stirred media mill. The influence of these parameters can be described by the so-called motion index. Up to a critical value of the motion index, the single bead follows basically the fluid flow and passes through the two zones of high energy density.

Stress intensity in stirred media mills and its effect on specific energy requirement

The comminution of limestone and fused corundum in stirred media mills has been investigated regarding specific energy requirement. The experimental results show that the tip speed of stirrer discs as well as the density and the size of grinding media affect the specific energy required to achieve a certain product fineness. Moreover, for comminution of fused corundum, the Youngs modulus of grinding media and of product particles influences the specific energy requirement. The kinetic energy of grinding media is transferred from the grinding media to the product particles during each stress event. For product materials with a high Youngs modulus (e.g., fused corundum), the transferred energy strongly depends on the Youngs modulus of grinding media, whereas the transferred energy is nearly independent of the Youngs modulus of grinding media for weak product materials. The effect of the operating parameters tip speed of stirrer, grinding media size, grinding media density and Youngs modulus of grinding media on the comminution result can be described by the stress intensity. The stress intensity is proportional to the energy, which is transferred from the grinding media to the product particles during a stress event. For each stress intensity, a certain relation between product fineness and specific energy exists. Therefore, the comprehensive influencing parameters on comminution of limestone and fused corundum in stirred media mills are the stress intensity and the specific energy.

SIMULATION OF A ROTARY DRYER FOR SUGAR CRYSTALLINE

Dynamic heat and material balances were developed, and residence time, heat and mass transfer rates were calculated using literature correlations. The model equations were solved numerically using the Speedup simulation package and tested against industrial data. Comparison of model predictions with industrial data show that the model is accurate for steady state operation and predicts dynamic trends that are consistent with engineering judgment. Predicted outlet moisture and temperatures differ by about ±10 % from the industrial data.

Morphology of Filamentous Fungi: Linking Cellular Biology to Process Engineering Using Aspergillus niger

In various biotechnological processes, filamentous fungi, e.g. Aspergillus niger, are widely applied for the production of high value-added products due to their secretion efficiency. There is, however, a tangled relationship between the morphology of these microorganisms, the transport phenomena and the related productivity. The morphological characteristics vary between freely dispersed mycelia and distinct pellets of aggregated biomass. Hence, advantages and disadvantages for mycel or pellet cultivation have to be balanced out carefully. Due to this inadequate understanding of morphogenesis of filamentous microorganisms, fungal morphology, along with reproducibility of inocula of the same quality, is often a bottleneck of productivity in industrial production. To obtain an optimisation of the production process it is of great importance to gain a better understanding of the molecular and cell biology of these microorganisms as well as the approaches in biochemical engineering and particle technique, in particular to characterise the interactions between the growth conditions, cell morphology, spore-hyphae-interactions and product formation. Advances in particle and image analysis techniques as well as micromechanical devices and their applications to fungal cultivations have made available quantitative morphological data on filamentous cells. This chapter provides the ambitious aspects of this line of action, focussing on the control and characterisation of the morphology, the transport gradients and the approaches to understand the metabolism of filamentous fungi. Based on these data, bottlenecks in the morphogenesis of A. niger within the complex production pathways from gene to product should be identified and this may improve the production yield.

Atomic force microscopy studies on the nanomechanical properties of Saccharomyces cerevisiae

In the past years atomic force microscopy (AFM) techniques have turned out to be a suitable and versatile tool for probing the physical properties of microbial cell surfaces. Besides interaction forces, nanomechanical properties can be obtained from force spectroscopic measurements. Analyzing the recorded force curves by applying appropriate models allows the extraction of cell mechanical parameters, e.g. the Youngs modulus or the cellular spring constant. In the present work the nanomechanical properties of the bakers yeast Saccharomyces cerevisiae are extensively studied by force spectroscopy using an AFM. Single cells deform purely elastically so that a cellular spring constant can reliably be determined. It is presented, how this spring constant depends on the probing position on the cell, and how it depends on the extracellular osmotic conditions. Investigations aiming a statistically firm description of the nanomechanical behavior of the yeast cell population are conducted. Finally, the informative value of the cellular spring constant as a cell mechanical parameter is critically discussed.