MODERNE PROZESS- UND PRODUKTGESTALTUNG AM BEISPIEL DER TROCKNUNG

Von Lebensmitteln bis hin zu Kunststoffgranulaten, von Mineralien bis hin zu Baustoffen, von Katalysatoren bis hin zu Pillen sind die meisten und wertvollsten materiellen Güter der heutigen Gesellschaft Feststoffe, die während ihrer Herstellung oder Handhabung in aller Regel getrocknet werden müssen. Dabei muß der Prozeß auf Wirtschaftlichkeit und Nachhaltigkeit, das Produkt auf Qualität und Mehrwert gestaltet werden. Welche immateriellen Güter – also welche wissenschaftlichen und technischen Herangehensweisen – hierzu beitragen können bzw. notwendig sind, versucht der Artikel anzudeuten

Experimental investigation of drying by pore networks: influence of pore size distribution and temperature

Isothermal and non-isothermal drying of pore structures has been experimentally investigated using 2D square network models of interconnected etched channels with different (Gaussian) distributions of the channel width. In experiments with imposed temperature gradients, the temperatures either increase from the open side of the network with increasing network depth (referred to as the positive temperature gradient) or the temperatures decrease with increasing distance from the network opening (i.e. a negative temperature gradient). Experiments reveal that the observed phase patterns, or the distributions of liquid and gas, during drying are significantly depending on the direction of the temperature gradient; but also the presence of macro channels can have a strong effect on the phase patterns as well as on drying time

Temperature gradient induced double stabilization of the evaporation front within a drying porous medium.

Drying of porous media very often occurs in the presence of significant temperature gradients because heat fluxes are imposed in many situations in order to decrease the drying time or to facilitate the moisture removal at a higher humidity of the surrounding gas phase. Here we consider the situation where the temperature increases with depth. We show from experiments with a micromodel that the temperature gradient induces the stabilization of the evaporation front within the model porous medium according to two different mechanisms occurring consecutively. The first mechanism occurs in the liquid phase and is explained by the dependence of surface tension upon temperature. This results in the preferential invasion of the warmer zones. The second mechanism occurs gas-sided due to the dependence of saturation vapor pressure upon temperature. We show that the time scales of both mechanisms are different leading to the temporary formation of distinctive phase patterns from which different periods of drying can be discriminated

A pore network model of drying with capillary liquid rings

Modelling of drying processes without adjustable parameters is still a challenge. As emphasized in several previous works,e.g. [1-3], this might be due to the impact of liquid films trapped in the corners of the pore space. In this study, we present and analyse a drying experiment with a micromodel which clearly shows the presence of corner films. In contrast with previous works, however, the corner films do not form a system of interconnected films extending over large regions. They rather form isolated capillary rings surrounding the solid blocks of the device. These capillay rings can be regarded as a quasi-two dimensional version of liquid bridges often observed in the contact regions between grains in soils and packings of particles, [4]. These capillary rings essentially remain confined in the two-phase region. As a result, their impact on drying rate is much smaller than in the systems favouring films hydraulically connected over long distances. The capillary liquid ring formation is taken into account in a pore network model of drying [5]. This model leads to a satisfactory agreement with the experiment provided that the lateral pinning of liquid phase observed in the experiment is included in the model. This model enriches the family of pore network models of drying and can be considered as a step toward the modelling of secondary capillary effects in drying in more complex geometry, such as a random packing of particles. The consideration of capillary rings in drying is important not only for a better prediction of evaporation rates but also for situations where the liquid phase contains particles or dissolved salt for example. For example, in the presence of salt, capillary rings are special locations where crystallization will take place as a result of ring evaporation. Since the capillary rings are likely to form in the contact regions between grains, they might play a role in the damages induced by the crystallization process

Dependency of continuum model parameters on the spatially correlated pore structure studied by pore-network drying simulations

[EN] Pore-network simulations are carried out for monomodal and bimodal pore structures with spatially correlated pore-size distributions. The internal and surface relationships between the partial vapor pressure and saturation as well as the moisture transport coefficient for these model porous structures are identified from the post-processing of the corresponding pore-network model solutions. The simulation results show that the deviation of the partial vapor pressure from the saturation vapor pressure in the presence of liquid – which is referred to as non-local equilibrium effect – in the bimodal pore structures is less pronounced than in the monomodal pore structures. For the monomodal pore structures the moisture transport coefficient profile is not unique over the entire drying process, whereas this profile depends marginally on the drying history of the bimodal pore structures. Finally the ability of the continuum model to predict the results of the pore-network simulations for multiple realizations of the pore space is assessed.This work was financed by the German Research Foundation (DFG) within the Graduate School 1554 “Micro-Macro-Interactions in Structured Media and Particulate SystemsLu, X.; Kharaghani, A.; Tsotsas, E. (2018). Dependency of continuum model parameters on the spatially correlated pore structure studied by pore-network drying simulations. En IDS 2018. 21st International Drying Symposium Proceedings. Editorial Universitat Politècnica de València. 307-314. https://doi.org/10.4995/IDS2018.2018.7417OCS30731

Fractal Phase Distribution and Drying: Impact on Two-Phase Zone Scaling and Drying Time Scale Dependence.

In an article published in 2008, Professor A.R. Mujumdar and his colleagues reviewed some applications of fractal concept on drying. As a modest continuation to this article, we give an overview on three drying-related issues where fractal aspects are present. First, we discuss within the framework of the theory of invasion percolation in a gradient the characteristic lengths that determine the extent of the hydraulically connected region during drying. It is pointed out that the scaling of this region is fundamentally different in 2D and in 3D, owing to the different percolation properties in 2D and 3D. In particular, it is shown that the fractal region only represents a small region of a drying front in 3D systems. Then a situation is described where fractal porous structures form as a result of an evaporation process. Finally, we consider drying in systems characterized by an initial fractal distribution of the liquid phase (invasion percolation cluster), a situation expected to happen in PEM fuel cells, and explore the size-dependent property of the overall drying time from pore network simulations

Multi-zone & multi-compartment model for dynamic simulation of horizontal fluidized bed granulator

[EN] Due to the ongoing development and implementation of process control and observation techniques in production processes of particulate products, the research on complexly designed process apparatuses has become of great interest. The work presented in this paper is focused on a model-based study on a multi-chamber horizontal fluidized bed apparatus for fluidized bed layering granulation. The model for the solid phase is extended by a new drying model. Because of the great variety of parameters that influence this complex system a preliminary model-based study on a simplified setup shall show which construction or process parameters influence the product quality.Financial support for this research from German Research Foundation (DFG) as part of the SPP project DynSim 1679 is gratefully acknowledged.Mielke, L.; Bück, A.; Tsotsas, E. (2018). Multi-zone & multi-compartment model for dynamic simulation of horizontal fluidized bed granulator. En IDS 2018. 21st International Drying Symposium Proceedings. Editorial Universitat Politècnica de València. 123-130. https://doi.org/10.4995/IDS2018.2018.7474OCS12313

Heat and mass transfer modelling of continuous Wurster-spray-granulation with external product classification

[EN] Wurster granulation with external product classification can be used for stable continuous coating or layering processes. It has been ascertained from recent population balance simulations that the ratio of the spray rate to the nuclei feed rate can be used to control the thickness of the sprayed product layer. However, thermal conditions are not considered by population balances regarding the particle size as distributed property. For this reason, heat and mass transfer is investigated in the present contribution by modelling of several subprocesses. The results can be used to discuss the cause of fluidized bed destabilization due to over-wetting.Mueller, D.; Bueck, A.; Tsotsas, E. (2018). Heat and mass transfer modelling of continuous Wurster-spray-granulation with external product classification. En IDS 2018. 21st International Drying Symposium Proceedings. Editorial Universitat Politècnica de València. 1383-1390. https://doi.org/10.4995/IDS2018.2018.7269OCS1383139

Drying of thin porous disks from pore network simulations

Thin porous media (TPM) are porous layers that are characterized by a small thickness, usually orders of magnitude smaller than the lateral dimensions. We present pore network simulation revealing that drying of TPM is substantially different from drying of thicker porous media because of the impact of the small thickness of only a few pore layers on the liquid phase structure during drying. The small thickness limits the long-distance connectivity within the liquid clusters and thus causes the formation of smaller clusters characterized by shorter residence times. As a result of this stronger and earlier liquid phase fragmentation the drying of TPM is shown to be significantly more sensitive to the distribution of the evaporation flux at the surface. It is also shown that the drying behavior transition from thin to thick porous media is progressive. Moreover, it is discussed how an imposed temperature distribution can be used to control the evolution of the liquid cluster distribution in a TPM and thereby the evolution of the evaporation rate