Clogging Phenomena Research Articles

Infiltration behavior and face stability of carbonate-added slurry shield tunnel

최근 연약지반 터널 현장에 활용되는 슬러리 쉴드 TBM은 굴진면으로 슬러리가 주입됨으로 그 안정성을 확보하는 공법이다. 하지만 간극이 과다하여 슬러리에 의한 폐색이 발생하지 않는 조립질 지반에서는 적용이 어렵기 때문에 첨가제를 혼입하여 사용하기도 한다. 본 연구에서는 첨가제의 역할로서 탄산가스를 주입함으로 슬러리가 주입되었을 때에 간극 내에 탄산가스가 흡착하여 폐색현상을 촉진시키는 효과를 규명하였다. 실내실험 결과 탄산 혼입에 따라 슬러리 쉴드 터널이 적용 가능한 유효입경이 1.0 mm에서 2.6 mm가량으로 증가하였고, 필터계수 <TEX>${\lambda}$</TEX>가 <TEX>$0.007sec^{-1}$</TEX>이상인 경우에 탄산에 의한 효과가 발생함을 알 수 있었다. Slurry shield tunnelling ensures stability by pressurizing the tunnel face with the slurry contained in the chamber. It resists water and earth pressure in order to prevent the failure in the tunnel face during tunnel excavation. If the ground is relatively coarse, slurry can not clog the tunnel face and excessive slurry infiltration will occur. In this case chemical compounds or additives should be added to the slurry in order to improve the clogging phenomena at the tunnel face. In this study, the effect of the carbon dioxide gas as an additive to the slurry instead of chemical compounds on the capability of enhancing the clogging in the tunnel face is investigated. Bubbles arising from the carbonate-added slurry are trapped in the soil voids enhancing the clogging capability. This effect is studied in this paper by performing laboratory model tests simulating in-situ conditions, and by adopting the fine particle clogging theory. Tunnel face stability analysis was also performed and it was found that the effective size (<TEX>$D_{10}$</TEX>) of soils which can guarantee tunnel stability utilizing the carbonate-added slurry increased from 1.0 mm up to 2.6 mm. Moreover, Stability analysis showed that the tunnel face is stable if the <TEX>${\lambda}$</TEX>(deposition coefficient) value is greater than <TEX>$0.007sec^{-1}$</TEX>.

Alumina coatings on carbon bonded alumina nozzles for active filtration of steel melts

Slip casted, carbon bonded nozzles with and without alumina based active coatings have been simultaneously tested in a novel casting simulator according to their clogging behavior against endogenous as well as exogenous non-metallic inclusions. Computer tomography images as well as SEM micrographs support the investigation of the clogging areas. The nozzle with the alumina active coating leads to increased clogging phenomena based on endogenous and exogenous inclusions as well as steel clogged particles. Correlating the clogging to a type of improved “filtration efficiency”, a higher filtration of steel melts is expected with the application of alumina active coatings on carbon bonded filters.

New method to assess tube support plate clogging phenomena in steam generators of nuclear power plants

Pressurized water reactor steam generators (PWR SGs) are large components whose main function is to cool the fission reactor by extracting the thermal power conveyed by the primary coolant, and thus to produce steam for the turbine generator. Tube support plate (TSP) clogging phenomena may occur in the SG when iron oxide particles carried in the secondary feedwater get unavoidably deposited inside the SG structure, and specifically in two main regions: on the U-tube outer walls and on the quatrefoil sections of the plates that support the U-tubes. They may reduce its cooling efficiency and impact its dynamic behaviour, thus leading to possible safety issues. Methods based on local inspections of the SG have been designed and are already in use. They produce estimators of the local TSP clogging ratio, during the yearly outage of the plant for refuelling and maintenance. A new method to assess a global estimator of the TSP clogging ratio is presented. This method is based on a 1D physical model of the SG that reproduces the complex dynamics of the two-phase flow phenomena inside the SG. The model was developed in the Modelica language. This model is used to compute response curves of the SG characteristics to a particular transient that challenges the dynamics of the SG, when affected by TSP clogging. The estimator is obtained by comparing the computed response curves to real response curves measured on-site. The method is still under validation. However, first results show that it is able to give global estimators that are consistent with local ones. This new method is expected to improve the monitoring of possible TSP clogging phenomena by producing quantitative estimators with better accuracy, with a quarterly periodicity while the plant is in operation.

Potential risk of calcium carbonate precipitation in agricultural drain envelopes in arid and semi-arid areas

Soil particle deposition and/or chemical precipitation can reduce the permeability of drain envelopes and filters. The first step in recognizing clogging phenomena is the identification of the nature of the precipitating materials. Calcium carbonate is a substance of low solubility that precipitates rapidly, forming a hard pan layer in the soil and/or clogging drain envelopes. The main objective of the present study is to investigate the precipitation risk of calcium carbonate in agricultural drain envelopes in the Khuzestan province of Iran. In this study, three indicators namely Ryznar, Langelier and Stiff-Davis indices were employed to assess the precipitation risk of calcium carbonate in agricultural drainage water. Results showed that all agricultural drainage systems in the study area give evidence of a potential risk of calcium carbonate precipitation, but the severity of the problem is different. The results also showed that the Ryznar and Stiff-Davis indices provide a better estimation of the potential precipitation risk of calcium carbonate than the Langelier index. Analyses of soil samples and drain envelopes from a drainage system , installed in the Abadan palm grove, showed that the main chemical component was calcium carbonate.

Contrasting Clogging in Granular Media Filters, Soils, and Dead-End Membranes

Clogging in porous media is a problem in environmental engineering, hydrogeology, soil science, and petrology. However, a comparison of the literature reveals qualitatively different clogging behavior in different porous media: in granular media filters, increasing clogging is associated with slower flow, more flocculated conditions, and smaller fractal dimensions. In soils and dead-end membranes, increasing clogging is associated with faster flow, more dispersed conditions, and larger fractal dimensions. This paper documents these differences, discusses them in light of two key intermediate variables, colloid accumulation and deposit morphology, then presents a new conceptual model that explains the reported clogging phenomena as a function of specific deposit, fractal dimension, and a new variable, deposit location. Testing this model is possible using recently introduced experimental techniques.

Constructed wetland clogging: A proposal for the integration and reuse of existing knowledge

Horizontal subsurface constructed wetlands (HFCWs) are examples of the use of appropriate technology for the sanitation of small communities. The most common problem of HFCWs is clogging; but the exact mechanisms and consequences of clogging in HFCWs are still unclear. This paper describes the methodology followed to acquire and analyse the available data and knowledge to amend the clogging phenomena in constructed wetlands. This manuscript also presents an attempt to integrate and reuse the information extracted from the literature and real experiences, to contribute to the prevention and solution of clogging problem. The management of this information allowed the proposal of an operation and maintenance protocol for clogging. The paper concludes with some recommendations to improve the performance of HFCW technology.

Clogging in intermittently dosed sand filters used for wastewater treatment

Clogging in intermittent sand filter (ISF) systems was analyzed using an unsaturated flow model coupled with a reactive transport model. Based on the results of a model sensitivity analysis, several variables were determined to be important in the clogging phenomena observed in ISFs, including hydraulic loading rate, influent chemical oxygen demand (COD) concentration, filter dosing frequency, and time of operation. Several modes of operation were identified that minimize the growth of bacteria at the filter surface. Following the sensitivity analysis, several case studies where ISF clogging was documented were simulated using the model. The results from the case study model simulations were found to be correlated with the total suspended solids loading rate (TSSLR) at the point of clogging. A model was developed that relates biomass development at the surface of ISFs with the TSSLR that can be sustained without clogging. The engineering significance of the model is presented in terms of operational and design considerations.

Effect of microstructure on blood cell clogging in blood separators based on capillary action

In this study, the blood cell clogging phenomenon occurring in blood separators based on capillary action is carefully investigated and how to minimize the reduction in plasma separation speed caused by clogging is discussed. Four different blood separators are fabricated on optically transparent glass substrates to clearly observe the blood plasma separation and the blood cell clogging in the microfluidic devices. Each separation experiment is captured by a high-speed video camera. The captured images are analyzed using a theoretical model proposed in this study to quantify the effect of the microstructure on the degree of blood cell clogging. Finally, design guidelines for the microstructure of the micro blood separator are discussed based on the analysis.

Clogging of Pervious Monolayers Subjected to Radial Flow

Particle migration within porous networks often results in permeability reductions due to clogging. In many field applications, fluids and entrained particles travel radially toward producing wells. The nonhomogeneous velocity field associated with radial flow produces particle-level force variations that in turn lead to spatially dependent pore-scale mechanisms and unique clogging phenomena. The fluid-driven migration and entrapment of particles in radial flow is studied experimentally in custom-built pervious monolayers that allow for direct visualization of clogging processes. Experimental results demonstrate that in particular hydrodynamic conditions, particle clogging evolves in the form of a self-stabilizing clogging ring that localizes at a characteristic radial distance that depends on the initial producing flow rate. Gradual clogging by surface deposition of particles is observed in less severe hydrodynamic conditions. The particle migration and clogging processes observed are analyzed by considering particle-level force balance through dimensionless parameters.

Relation of haemofilter type to venous catheter resistance is crucial for filtration performance and haemocompatibility in CVVH—an in vitro study

Main factors for the overall performance of haemofilters (HF) are membrane features and filter durability without clogging/clotting of capillaries. However, the venous line resistance (Pv) is a powerful force for net filtrate flux resulting in haemoconcentration and thus is enhancing the phenomenon of membrane clogging. Therefore, we hypothesized that catheter type, as it is associated with Pv-levels, contributes to the extent in which filter longevity and filtration performance are restricted due to blocked hollow fibres. Heparinized porcine blood (5 IU/ml) was circulated in an in vitro system for haemofiltration (FH6S-filters were used, Ca. Gambro). Three different sizes of catheters for peripheral vein access (Vygonuele V, Ca. Vygon) were alternately inserted into the circuit for blood return from the filter to the reservoir. To produce Pv-levels lower than commonly induced by Shaldon catheters, a 14G-vygonuele was used. Pv-levels standard for 11-12 French catheters were provided by using a 16G-vygonuele. To produce Pv-levels common for low-French or tri-lumen catheters, a shortened 18G-vygonuele was used. The respective Pv-levels attained were compared with respect to the overall filtration performance (system pressures, haemocompatibility and sieving coefficients). Catheters of 14 and 16G enabled transiently maximal blood flow (Qb)/filtration rates (Qf) of 300/60 ml/min and continuous filtration with Qb/Qf of 200/40 ml/min. The shortened 18G catheter reduced maximal flow rates down to Qb/Qf of 200/40 ml/min, and continuous flow rates down to Qb/Qf of 125/25 ml/min. At the end, median values for blocked hollow fibres were, 35% in the 14G-group, 40% in the 18G-group and 70% in the 16G-group. Haemocompatibility appeared to be higher in the 14G-group with respect to various parameters when compared with the other groups. The flow resistance by the catheter chosen for haemofiltration clearly influenced the filtration performance. Thus, investigations focused on compatibility of catheter type as it related to Pv-levels with the particular method of renal replacement therapy that should be performed. This point could be crucial in reducing filter clogging and haemostasis during CVVH.

Observation Evaluation of Nozzle Clogging in a Micro-orifice Impactor Used for Atmospheric Aerosol Sampling

ABSTRACT A micro-orifice impactor uses micro-orifice nozzles to extend the cut sizes of the lower stages to as small as 0.18 μm in diameter without resorting to low pressures or creating excessive pressure drops across the impactor stages. In this work, the phenomenon of nozzle clogging caused by particle deposition was investigated experimentally for a commercial micro-orifice uniform deposit impactor (MOUDI, MSP model 100). Using an optical microscope, we observed that the micro-orifice nozzles of the lower three stages were partially clogged due to particle deposition during the atmospheric aerosol sampling. To examine the effect of nozzle clogging on the performance of the impactor, the pressure drop and the particle collection efficiency were evaluated for the lower three stages. The pressure drops across the clogged nozzles were higher than the nominal values given by the manufacturer. The particle collection efficiency of each stage was evaluated by using an electrical method for fine particles with diameters in the range of 0.1–0.6 μm. Monodisperse liquid dioctyl sebacate (DOS) particles were used as test aerosols. A Faraday cage was employed to measure the low-level current of the charged particles upstream and downstream of each stage. The collection efficiency curves shifted to correspondence to smaller orifice sizes, and the 50% cutoff sizes were much smaller than those given by the manufacturer for the three stages with nozzles less than 400 μm in diameter.

Anhydrite cementation and compaction in geothermal reservoirs: Interaction of pore-space structure with flow, transport, P– T conditions, and chemical reactions

In order to analyze local clogging phenomena due to the precipitation of anhydrite in the pore-space we performed a petrophysical study on cores and data from three boreholes probing the Rhaetian hydrothermal aquifer in northern Germany (Allermöhe, Neuruppin 1/88, Neuruppin 2/87). The pore size geometry of sandstones was studied using, among other methods, pulsed field gradient-nuclear magnetic resonance (PFG-NMR). We found two facies types with clearly different diagenetic history: (1) a fine-grained sand with small pores, mechanically compacted during diagenesis into an average sandstone of fractal pore geometry; (2) a coarser sand with larger pores, almost completely cemented by anhydrite after compaction had reduced the pore-space to a porosity of 30%; thus no further mechanical compaction occurred and the pores remained smooth. In contrast, the fine-grained facies was not cemented, possibly because anhydrite crystal nuclei are unstable in pores of insufficient size. With regard of these two facies types we developed: (1) an approach for calculating vertical logs of anhydrite and permeability content from gamma density measurements in boreholes or on core; (2) a chemical reaction model to calculate the amount of precipitation and dissolution of each mineral species at high temperature and salinity. These petrophysical and geochemical models are integrated into the numerical simulation tool SHEMAT for simulating coupled flow, heat and species transport, and chemically induced permeability changes. A comparison of the diagenesis of the cemented and un-cemented facies types yields an improved understanding of the geological conditions required for anhydrite cementation. Specifically, with regard to the studied Rhaetian sandstones, numerical reactive flow simulations demonstrate that intense, local anhydrite cementation may occur when saline formation waters mix with hot brines ascending on faults.

The impact of sewage composition on the soil clogging phenomena of vertical flow constructed wetlands

The infiltration rate and therefore the principal function of a sand based vertical flow constructed wetland (VFCW) is influenced by the content of suspended solids (SS) and chemical oxygen demand (COD) of the waste water supply. In this study there were three operating conditions defined as "No Clogging"; "Partly Clogging" and "Clogging". Investigations on 21 VFCWs approved analytical differences between these conditions. The content of SS and especially particles > 50 microm are considered to play a key role. These particles are of the same size as the pores in which seepage mainly occurs. Thus their potential for surface blocking is high. It is concluded that the construction and size of the primary settling has to ensure that the mean concentration of SS after settling does not exceed 100 mg l(-1). The results of this study indicate that the area of the VFCW should be designed for a maximum loading rate of 5 g m(-2) d(-1) and the COD load should not exceed 20 g m(-2) d(-1).

Biogeochemical Evaluation of Mechanisms Controlling CaCO3(s) Precipitation in Landfill Leachate-Collection Systems

A common failure mode for landfills is clogging of the leachate-collection system. The reduction in hydraulic conductivity associated with clogging causes a buildup of leachate head on the underlying liner, potentially increasing advective contaminant transport from the landfill and contaminating adjacent groundwater. In this paper, the biogeochemical model CCBATCH is used to link a primary cause of leachate collection system failure—CaCO3(s) precipitation—to anaerobic degradation of volatile fatty acids (VFAs) in column reactors used to study the clogging phenomena. One key to applying CCBATCH correctly was dividing the VFA conversion into two steps: conversion of propionate to acetate, carbonic acid, and methane; and acetate conversion to methane and carbonic acid. The primary driver for CaCO3(s) precipitation in the columns was acetate fermentation to CH4 and H2CO3, which increased the total carbonate concentration in the leachate and shifted the acid/base control to a weaker acid system, which caused an increase in solution pH. A second key to proper modeling was adding CO2(g) gas transfer to CCBATCH. The modeling results indicate that the kinetics of CO2(g) gas transfer was a key control over leachate chemistry once acetate fermentation was nearly complete. These results suggest that the best approach for the long-term control of CaCO3(s) clogging may be to enhance CO2(g) gas transfer from the leachate while buffering the leachate pH to near neutral. Taken together, these actions should decrease the yield of CaCO3(s) precipitated per mass of acetate removed.

노즐 막힘이 미세 오리피스형 다단 임팩터의 입자 채취 성능에 미치는 영향

A cascade impactor is a multistage impaction device used to separate airborne particles into aerodynamic size classes. A micro-orifice impactor uses micro-orifice nozzles to extend the cut sizes of the lower stages to as small as 0.05 in diameter without resorting to low pressures or creating excessive pressure drops across the impactor stages. In this work, the phenomenon of particle clogging in micro-orifice nozzles was experimentally investigated for a commercial micro-orifice uniform deposit impactor (MOUDI). It was observed, using an optical microscope, that the micro-orifice nozzles of the final stages were partially clogged due to particle deposition during the aerosol sampling. Therefore the pressure drops across the nozzles were higher than the nominal values given by the manufacturer. To examine the effect of particle clogging in micro-orifice nozzles, the particle collection efficiency of the MOUDI was evaluated using an electrical method for fine particles with diameters in the range of 0.1-0.6 . The monodisperse liquid dioctyl sebacate (DOS) particles were used as test aerosols. A faraday cage was employed to measure the low-level current of the charged particles upstream and downstream of each stage. It was found that the collection efficiency curves shifted to correspond to smaller orifice sizes, and the 50-% cutoff sizes were much smaller than those given by the manufacturer for the three stages with nozzles less than 400 in diameter.

Removal of Toluene in Gas Streams by a Fibrous-Bed Trickling Filter

A novel fibrous-bed trickling filter was developed to remove toluene present in contaminated air. Pure culture of Pseudomonas putida F1 was attached on fibrous-bed and utilized toluene as the carbon source. Experimental results indicated the removal efficiency decreased with the increase of inlet concentration. In general, the removal efficiency of toluene was greater than 90% when the inlet loading capacity was below 70 g m−3h−1. The elimination capacity increased with increasing inlet loading capacity, but the increased rate decreased gradually. When the inlet loading capacity increased to 300 g m−3h−1, the elimination capacity could approach to 130 g m−3h−1. The first order kinetics model was useful to describe the removal of toluene in this filter and an excellent linear relationship was found between the apparent first order parameter and inlet concentration (ranging from 1.2 g m−3 to 3.5 g m−3). Also, the performance of fibrous-bed trickling filter was relatively stable during the four-month period of continuous operation. Slight clogging phenomena of filters were observed only under high loading capacity.

Modelling of the Interaction Between an Engineered Clay Barrier and Concrete Structures in a Deep Storage Vault

AbstractThe degradation of concrete structures in a storage vault will release an alkaline plume influencing the geochemical evolution of clay engineered barriers or plugs. The studied configuration has a representative scale and is composed of a rectangular clay barrier interfaced on one side with a cement block and on the other side with an external granitic field. A clay model including major and accessory minerals is used; cement material is supposed to be made of portlandite and CSH minerals. The clay barrier and the cement block are assumed to be initially saturated and in equilibrium with their respective interstitial water. Diffusive transport of aqueous species coupled to chemistry is simulated in a one dimension space up to 10000 y. The feed-back of the porosity evolution on the transport properties is not taken into account. The temperature is supposed to be constant (25°C). Calculations are performed with the TRIO-EF coupled transport-chemistry code.The main results deal with the evolution of the pH profile across the clay barrier, the dissolution of clay minerals and the precipitation of new solid phases in the clay such as albite, gibbsite and CSH. These simulations show that the pH excursion includes only a minor part of the clay barrier at 10 000 y and that the CSH precipitation retards the alkaline plume progression. The appearance of local accumulations of newly precipitated minerals might reduce strongly the porosity. Future developpements of this work include: (i) the completion of the geochemical model by introducing other minerals in concrete (other CSH phases and sulfo-aluminates) and ion exchange sites within the clay, (ii) the implementation of a precipitation/dissolution model in order to describe on a realistic way the porosity profile evolution and possible clogging phenomena.

Role of the Clogging Phenomena in Erroneous Implications of Conventional Data Analysis for Constant Pressure Cake Filtration

Constant pressure filtration of kaolin slurries is studied by directly measuring the pressure drop across the filter medium under conditions leading to various extents of medium clogging. Experimental data are analyzed in terms of the conventional filtration theory based on the two-resistances-in-series model. The filter medium resistance and the cake resistance values obtained by the conventional data interpretation techniques, which require only the measurement of total filtration pressure, are compared withthose calculated from directly measured pressure drops across the filter medium and the cake. The results show that the conventional analysis yieldsunrealistic filter medium resistances if the medium clogging is significant and continues throughout the course of filtration. The medium resistanceis a significant part of the total filtration resistance even at long filtration times. The average specific cake resistance obtained by the conventional methodology does not appear to be in error when compared to its value calculated from the directly measured cake resistance at long filtrationtimes. An actually incompressible cake, however, appears to be compressible due to cake clogging which increases with increasing filtration pressure.

Gradient and Percolative Clogging in Depth Filtration

The phenomenon of clogging in depth filtration is investigated, in which a dirty fluid is "cleaned" by the trapping of dirt particles within the pore space during flow through a porous medium. This gives rise to a self-generated gradient percolation process which exhibits a power law distribution for the density of trapped particles at downstream distance x from the input, both in idealized and lattice networks. Implications for efficient filter design are also mentioned.

Soil clogging phenomena in constructed wetlands with subsurface flow

The paper presents a short description of soil clogging processes during sewage infiltration as well as a simple theoretical model of sand clogging by suspended solids. The Kozeny-Carman equation was used to investigate the influence of the decreasing porosity due to clogging on the sand permeability. A comparison of the results obtained using the derived theoretical relationships with empirical data showed an acceptable agreement.