Clogging Behavior Research Articles

Processes in vertical-flow reed beds: nitrification, oxygen transfer and soil clogging

Vertical-flow reed beds (VF) with intermittent feeding are extremely reliable regarding aerobic processes. For a save operation with high nitrification rates and without soil clogging it is essential to preserve aerobic conditions in the filter. The challenge is to keep aerobic conditions in the filter without oversizing the system (economical aspects). It is very difficult to determine the current oxygen content in the filters because it ultimately results from complex interactions of a large number of different influencing parameters such as loading rate, degree of clogging, temperature, and hydraulic behaviour of the reed bed. To gain better knowledge of this complex system, different tests and examinations were carried out over several years. Focusing on the questions of identification and the description of conversion and transport processes (water/gas), a full-scale treatment plant under clogged and non-clogged conditions was investigated in detail. Additionally soil column test were carried out. The results make it possible to describe some of the processes and their interactions in the filter body. Recommendations for a safe and controlled operation can be derived.

Imbibition test in a clay powder (MX-80 bentonite)

The MX-80 bentonite is a reference material for studies in clay barriers. This paper aims to present an investigation of the behaviour of such a material during an infiltration test from the bottom in oedometric conditions. Using dual-energy γ radiation technique, time variations of moisture content are measured at various locations along the clay sample. The transport of water occurring in this experiment can be described by the classical diffusion equation in terms of the Boltzmann variable. A parameter of hydraulic diffusivity is obtained with the Matano's method. Two fluid transport mechanisms govern the phenomena: imbibition of the microporosity in the clay grains and capillary imbibition in the mesoporosity around the grains. The competition between these two mechanisms will be put into relief by modelling the distribution of the water between and inside the bentonite grains. A clogging behaviour of the mesopores during the imbibition process will allow to explain the surprising decrease in hydraulic diffusivity.

Nozzle Clogging Behaviour of Ti-bearing Al-killed Ultra Low Carbon Steel

In the present work, attempt has been made to investigate the influence of Ti on the castability of Al-killed ultra low carbon (ULC) steel, and to establish the possible reasons for the inferior castability of the Ti-bearing ULC steel vis-a-vis Ti-free grades. The work also attempted to identify suitable counter-measures for prevention of inclusion deposition in the submerged entry nozzles (SEN) during continuous casting of Ti-bearing Al-killed ULC steels. Characteristics of inclusions in the submerged entry nozzle deposit and corresponding liquid steel and slag samples from RH degasser and tundish were investigated. The presence of small quantity of Ti-bearing alumina inclusion was identified to be responsible for the extensive melt freezing inside the SEN deposit and poor castability of Ti-bearing Al-killed ULC steels. The influence of all materials contacting liquid steel subsequent to the RH degassing, on the reoxidation behaviour and castability of liquid steel, was assessed from their chemical composition. The present investigation indicated that while reoxidation is bad for the castability of all Al-killed steels, it becomes worse in the case of Ti-bearing Al-killed steels, primarily due to the formation of Ti-bearing inclusions that promote large-scale melt freezing inside the nozzle deposits.

NOZZLE CLOGGING PREDICTION IN CONTINUOUS CASTING OF STEEL

Submerged entry nozzle connects the tundish and the mold in the continuous casting of steel. Continuous casting is usually done in series including 3–6 successive heats. Casting of a heat takes about 35–50 minutes. The nozzle is changed after each series and the new series is started with a new nozzle. About 360–720 tons of steel goes through a nozzle during its lifetime.The casting speed and the stopper rod position give the indication of nozzle clogging, but they cannot, however, answer the question, how long time the casting can continue and when the nozzle should be changed. In this paper, feedforward neural networks with backpropagation training were used in modelling the nozzle clogging behaviour at Rautaruukki Steel mill, at Raahe site.

Modelling pressure drop in hepa filters during dynamic filtration

A new model has been developed to predict the clogging behaviour of high-efficiency particulate air filters. The model takes into account the heterogeneous deposit of particles inside the filter medium. The filter is considered to be a series of elementary slices which are assumed to be homogeneously loaded by aerosol. Particles are assumed to form dendrites which can be considered as newly formed fibres. For each time increment and for each slice, particle collection efficiency of initial and new fibres is calculated according to the Payet model in order to give an overall prediction of the mass of aerosol deposited and the pressure drop. Model results are compared with experimental results.

Bacterial clogging of porous media: A new modelling approach

A comparison is made between existing mathematical models and experimental data that relate the reduction of the saturated hydraulic conductivity (K) of a porous medium to the porosity reduction caused by microbial growth. The models yielded a realistic prediction of a data set obtained with a model porous medium consisting of millimeter‐size glass spheres, but failed to predict the clogging behaviour observed in smaller‐than‐1‐mm sand. A new modelling approach, semi‐mechanistic in nature, is proposed that gives good predictions of fine sand media as well. It relaxes the assumption about uniformly‐thick biofilms by allowing a second arrangement to occur, i.e. discrete plugs filling the pore lumen. The new model requires input data on two intrinsic properties of the system, which renders it sufficiently flexible as to fit very different data sets. The two model parameters are Kmin, the minimum K value when all porosity is filled with microorganisms, and Bc, the biovolume fraction at which most cell detachment from biofilm occurs.

Long-term clogging behaviour of non-woven geotextiles with silty and gap-graded sands : Bhatia, S K; Qureshi, S; Kogler, R M In: Geosynthetic Testing for Waste Containment Applications (Papers to a Symposium Presented at Las Vegas, 23 January 1990)P285–298. Publ Philadelphia: ASTM, 1990 (ASTM Special Technical Publication N1081)

Long-term clogging behaviour of non-woven geotextiles with silty and gap-graded sands : Bhatia, S K; Qureshi, S; Kogler, R M In: Geosynthetic Testing for Waste Containment Applications (Papers to a Symposium Presented at Las Vegas, 23 January 1990)P285–298. Publ Philadelphia: ASTM, 1990 (ASTM Special Technical Publication N1081)

Long-Term Filtration Behaviour of Soil-Geotextile System

Abstract In addition to soil retention and permeability requirements for a geotextile filter, a geotextile should be able to function without having a significant number of its openings clogged. In order to evaluate the long-term filtration behaviour of a soil-geotextile system, a permeameter is developed in which both the long-term flow rates as well as the gradient ratio tests can be conducted simultaneously for different soils at different densities. Uniformly graded Ottawa sand mixed with various percentages of fine-grained silty soil is used to represent the unstable soil condition for internal soil migration. Two types of needle-punched nonwoven and one type of woven geotextiles are used whose opening sizes satisfy the soil retention criteria for the soil mixtures used. An unstable flow behaviour is observed during the initial period of filtration. Fine soil particles move toward the geotextile and are either trapped within the geotextile or retained upstream, decreasing the flow rate of the soil-geotextile system. Continuous reduction in flow rate is observed which finally reaches a constant stabilized value. It is also observed that the fine soils adjacent to the geotextile pass through it and the flow rate increases. The duration of such unstable behaviour and time taken to attain a stabilized flow condition increase with fine contents. The gradient ratio also attains a stable value in the long run after the formation of a soil filter cake in the soil-geotextile system. The initial unstable period does not indicate true clogging behaviour of the system. In view of this, it is recommended that a long-term stabilized gradient ratio be determined instead of the 24-h gradient ratio.

On the filtration of sodium oxide aerosols by multilayer sand bed filters

Multilayer sand bed filters are developed and the clogging and penetration behaviour of sodium oxide aerosols in these filters with flow rates up to 5 cm/sec and Reynolds numbers between 1 and 10 are investigated. Sand bed filters were optimized to take up high amounts (500 g/m 2) of chemically aggressive aerosols at reasonable pressure differences with efficiencies as good as Hepa-filters. Besides experimental results a mechanism explaining the clogging and penetration of sand bed filters is discussed.