Achim Dittler

Patchy cleaning of rigid gas filters - Transient regeneration phenomena : comparison of modelling to experiment

Abstract Rigid ceramic filter media, widely used for the removal of particles from gas streams at elevated temperatures tend to show patchy cleaning when the filter regeneration is incomplete [Filtr. Sep. (1989) 187]. In order to investigate the regeneration behaviour and the operational performance of partially regenerated, rigid gas cleaning filter media over many filtration cycles, experiments were performed in a filter test rig. The regeneration behaviour of the filter sample was characterized by the overall regeneration efficiency, the local frequency of regeneration, and the number and size of regenerated filter areas. Using only four adjustable parameters, our modelling results compare favourably with our experimental results, at room temperature. This favourable comparison of the regeneration behaviour between modelling and experiment is achieved only if it is assumed that cohesive and adhesive bonds, which are broken during filter regeneration, do not heal during the next filtration cycle. Assuming otherwise would cause (i) the dust cake to be removed at the same positions during every regeneration and (ii) the patch size to increase from cycle to cycle instead of decreasing as seen in the experiment. Therefore, the model development was guided by our extensive experimental results. This agreement of modelling with experiment indicates that the modelling has real predictive capabilities for operational filter cleaning. Both filter conditioning and dust cake compression significantly influence the operational performance of partially regenerated filter media.

Optical in situ measurement of dust cake thickness distributions on rigid filter media for gas cleaning

Abstract Rigid barrier filters are one of the options for particulate removal from gas streams by means of surface filtration. Non-uniform gas flow through the filter, possibly caused by inhomogeneities from the manufacturing process, affects the filtration behavior of such media, is suspected of causing an inhomogeneous dust cake build-up, and may influence regeneration adversely, resulting in an instable operating cycle. So far little is quantitatively known about the distribution of cake properties (e.g., porosity, thickness, etc.) across the filter surface, possible variations of this distribution over a number of filtration cycles, and how such variations may be connected with the distribution of residual cake patches after regeneration. This paper introduces an optical in situ technique to measure dust cake height distributions across a flat area of filter surface of 14 cm in diameter. Distributions are recorded by an image acquisition process with a lateral resolution of about 280 μm (1 pixel) and a vertical resolution of about 50 μm. Sample data of a partially regenerated ceramic filter medium are presented to illustrate the capabilities of the technique. The experiments were performed with quartz dust at ambient temperature and pressure on a filter test rig according to VDI Standard 3926 [VDI Richtlinie 3926, Prufung von Filtermedien fur Abreinigungsfilter, VDI Handbuch Reinhaltung der Luft, Band 6, 1991]. The data can be used to determine point-by-point growth rates of the filter cake during filtration and residual cake thickness after pressure pulse or reverse gas flow regeneration.

Simulation of operational behaviour of patchily regenerated, rigid gas cleaning filter media

Abstract Particle removal from hot process gases is frequently accomplished with regenerable ceramic filters. When regenerating such media periodically, the dust cake may be detached from parts of the filter surface while other regions remain intact (‘patchy cleaning’). The filtration process depends on how these patterns of incomplete regeneration evolve over a number of cycles, how they change the build-up of the new cake, and how they affect the pressure drop. A two-dimensional quasi stationary flow model is used to predict pressure drops as a function of regeneration efficiencies and regeneration patterns, taking into account the finite thickness and flow resistance of the medium itself. The effect of non-uniform cake build-up on the pressure rise during a filter cycle is also modelled for a partially regenerated filter. The calculations prove that the pressure drop rises faster for lower regeneration efficiencies and that also cycle times become briefer with lower regeneration efficiency. It can also be shown, that the regeneration pattern only influences the pressure drop curve at the very beginning of the filtration cycle but does not influence the filtration cycle times.

Measurement and simulation of the vacuum contact drying of pastes in a LIST-type kneader drier

Abstract The drying of paste-like products is a common industrial task. Usually a batch vacuum contact drying process is used and the process is carried out in a kneader dryer with a low stirrer speed due to very high torques appearing during the drying process. The process can be divided into two parts, the paste and the particulate regime. Both regimes are separated by a sharp maximum in the torque curve. The position of the maximum depends on the type of product. In the paste regime the drying rate decreases with decreasing moisture content of the product, while torque increases and the product temperature remains nearly constant. In the particulate regime the drying rate decreases further with decreasing product moisture content. At the same time product comminution takes place, the torque decreases and the product temperature rises. Drying rate curves are measured for aqueous suspensions of china clay using a batch kneader drier at pilot plant scale. The influence of the main drying parameters on the drying process is investigated experimentally and theoretically. The measured drying rate curves are compared to calculations based on the penetration model for contact drying. Neglecting mass transfer effects and assuming that the particle size distribution of the product remains constant, the penetration model is able to describe both the paste regime and the particulate regime of the drying process over the whole moisture range. Measured and calculated drying rate curves are in good agreement for a variety of drying parameters.

The influence of conditioning and regeneration on the separation behaviour of rigid surface filters for the separation of particles from gases

Abstract Rigid ceramic filter media can be used for the separation of particles from gas streams at elevated temperatures. In order to characterize comparatively the separation behaviour of differently structured filter media over a multitude of filtration cycles, experiments were performed in a filter test rig. The filter test rig used is built in accordance with VDI guideline 3926 and equipped with a special type of optical particle counter, which has the advantage of measuring both the particle size and the particle concentration simultaneously and in situ on the clean gas side. It is demonstrated that by far largest share of the particles reaches the clean gas as a result of the filter regeneration process. During the subsequent formation of the dust cake, the particle penetration is almost zero. The regeneration parameters, actually in essence only the tank pressure, possess the decisive influence on the separation behaviour. In the course of a filtration experiment, the number of particles reaching the clean gas reduces with the increasing number of filtration cycles due to the filter conditioning. The particles arriving in the clean gas during regeneration are extremely fine. The mean particle size is almost entirely independent of the regeneration conditions and the filter cycle number. The fine particulate emissions measured here are an anthropogenic source of particulate matter in ambient air. They have to be considered as relevant with respect to the new standards of ambient air quality (PM 10 and PM 2.5). It is shown that membrane-coated ceramic filter media at identical operating conditions exhibit, as expected, a better separation and regeneration behaviour than fibrous ceramic, open-pored filter media. However, the advantages are at the expense of a 10 times higher pressure loss, which is essentially attributed to the membrane layer.

The Application of Diesel Particle Filters—From Past to Present and Beyond

AbstractsDiesel particle filters are primarily applied to remove particulate matter from the exhaust gas of diesel engines. They have become a standard component in exhaust aftertreatment systems when it comes to meeting the latest legislative emission targets. In systems currently applied coated diesel particle filters represent one component among a few others. Filters are combined with diesel oxidation and SCR-catalysts in order to reduce particulate and gaseous emissions in one system. Based on a brief description of particulate traps, as applied in buses and diesel cars in the 1980s, historical engineering challenges of the application are highlighted. Material/coating durability aspects, the deposition of high amounts of ash, use of low quality fuel and exhaust temperature restrictions for soot removal, as they were given back in the 1980s, are outlined. The paper describes technological key enablers, such as the availability of low-sulphur diesel fuel, which allowed the integration of particle filters downstream of an oxidation catalyst (DOC). Functionally a continuous soot removal can be achieved, limiting the necessity of fuel-consuming active regeneration events. In addition the paper discusses sources of ash and how the application of low-ash oils can lower the amount of ash accumulated in the filter system significantly. Finally future challenges of the particle filter application as a standard component in aftertreatment systems of combustion engines are outlined. Among these are the reactivity of soot, originating from the use of alternative fuels as well as the use of fuel with higher Sulphur content.

Rußfilter oder Aschefilter? – Grundsatzuntersuchungen zur Filtration des Abgases von Nutzfahrzeug-Dieselmotoren

Das Verstandnis der sog. Ascheproblematik in Dieselpartikelfiltern ist eine wesentliche Herausforderung bei der Dimensionierung des Partikelfilters fur die Anwendung zur Abgasnachbehandlung in Nutzfahrzeugen. Es ist bekannt, dass die Ascheablagerungen in Partikelfiltern hauptsachlich durch anorganische Bestandteile des Motorols bedingt sind. Die wirtschaftliche Auslegung des Partikelfilters erfordert eine periodische Entfernung der Asche aus den Partikelfiltern nach einer (herstellerspezifischen) Laufleistung ab etwa 270.000km. Zur Entfernung von Asche aus Partikelfiltern haben sich Verfahren der Ruckspulung mit einem Fluid etabliert. Druckluftbasierte Reinigungsverfahren liefern gute Reinigungsergebnisse, die aber nicht immer reproduzierbar zuverlassig erreicht werden konnen. Eine Verbesserung des Reinigungserfolgs kann durch die Ruckspulung der Filter mit Flussigkeit erzielt werden. Dabei kann ein maximaler Reinigungserfolg dann erzielt werden, wenn eine auf die Aschezusammensetzung abgestimmte Reinigungslosung verwandt wird. Damit ist die nahezu vollstandige Entfernung von Asche aus Partikelfiltern wiederholbar moglich. Letztlich bleibt die Vision eines Schmierols ohne anorganische Bestandteile. Die Ascheproblematik wurde sich deutlich einfacher darstellen. Die Auslegung von Partikelfiltern fur die Anwendung zur Abgasnachbehandlung in Nutzfahrzeugen ware als Lebensdauerbauteil ohne die Notwendigkeit einer aufwandigen Reinigung des Filters moglich.