Contact Time Distribution Research Articles

Enhancing ozonation technology: A case study on full-scale airlift reactor upgrades in the existing wastewater treatment plant's ozonation tank using CFD

Traditional pneumatic bubble column reactors (PBCRs) in wastewater treatment often have limitations in ozone distribution and disinfection efficacy. This study introduces an optimized Airlift Reactor (ALR) design, enhanced with Computational Fluid Dynamics (CFD), to improve ozone contact time and uniform distribution. Tailored for the SONATRACH GP1/Z Complex in Bethioua, Algeria, the ALR addresses the treatment needs of the facility's WWTP Phase-1, handling domestic wastewater. The study is unique for its full-scale implementation of the ALR, as previous research focused on lab-scale cases. Advanced CFD simulations optimized the ALR design, focusing on baffle configurations to enhance ozone disinfection. The upgraded One Side Baffled Airlift (OS-BALR) variant achieved a Total Coliform inactivation rate of 94.52 %, compared to 74 % with the traditional design. The OS-BALR also demonstrated a lower ozone mass transfer coefficient (KLa) of 0.0048 s−1, improving treatment efficiency and reducing energy consumption. While the Baffled Bubble Column (BBC) reactor showed a higher KLa of 0.0055 s−1 and a microbial removal efficacy of 96.12 %, it is more energy-intensive. This study advances wastewater treatment by bridging innovation with industrial application, contributing to sustainable wastewater management practices globally.

MSN‐CDF: New community detection framework to improve routing in mobile social networks

SummaryIn mobile social networks (MSN), portable devices of mobile users communicate with each other. The contact and intercontact times are the two most important parameters that specify the behavior of MSN. Many social features such as similarity, centrality, closeness, and distance are derived from contact and intercontact time patterns. Those parameters and features can be used in the community detection process in community‐based routing as a vector weight of network graph edges. But there is no solution for proper exploitation of them as a multicriteria optimization of community detection algorithms so that the communities become more adaptive with the MSN quiddity and the computational complexity of the algorithms to be stay constant. In this paper, we propose the MSN‐adaptive community detection framework, MSN‐CDF, which meets these demands. We improve the quality function of community detection methods, for example, the Intensity in clique percolation methods and the Modularity in modularity‐based methods, through our new model‐based joint optimization of their quality functions. To this end, to express the MSN contact and social features, we develop a criterion called MCSF. It holds the contact time and intercontact time distributions parameters and implies their social features for MSN purposes. This criterion has as little traffic as possible and enjoys from the probabilistic joint distribution model. The simulations show that MSN‐CDF reduces the network overhead significantly compared with the DiBUBB, Epidemic, and Dynamic Routing. Moreover, it outperforms DiBUBB and Dynamic Routing in terms of delivery rate and delivery latency.

What You Lose When You Snooze

In opportunistic networks, putting devices in energy-saving mode is crucial to preserve their battery, and hence to increase the lifetime of the network and foster user participation. A popular strategy for energy saving is duty cycling. However, when in energy-saving mode, users cannot communicate with each other. The side effects of duty cycling are twofold. On the one hand, duty cycling may reduce the number of usable contacts for delivering messages, increasing intercontact times, and delays. On the other hand, duty cycling may break long contacts into smaller contacts, thus also reducing the capacity of the opportunistic network. Despite the potential serious effects, the role played by duty cycling in opportunistic networks has been often neglected in the literature. In order to fill this gap, in this article, we propose a general model for deriving the pairwise contact and intercontact times measured when a duty cycling policy is superimposed on the original encounter process determined only by node mobility. The model we propose is general, i.e., not bound to a specific distribution of contact and intercontact times, and very accurate, as we show exploiting two traces of real human mobility for validation. Using this model, we derive several interesting results about the properties of measured contact and intercontact times with duty cycling: their distribution, how their coefficient of variation changes depending on the duty cycle value, and how the duty cycling affects the capacity and delay of an opportunistic network. The applicability of these results is broad, ranging from performance models for opportunistic networks that factor in the duty cycling effect, to the optimisation of the duty cycle to meet a certain target performance.

Two loci single particle trajectories analysis: constructing a first passage time statistics of local chromatin exploration

Stochastic single particle trajectories are used to explore the local chromatin organization. We present here a statistical analysis of the first contact time distributions between two tagged loci recorded experimentally. First, we extract the association and dissociation times from data for various genomic distances between loci, and we show that the looping time occurs in confined nanometer regions. Second, we characterize the looping time distribution for two loci in the presence of multiple DNA damages. Finally, we construct a polymer model, that accounts for the local chromatin organization before and after a double-stranded DNA break (DSB), to estimate the level of chromatin decompaction. This novel passage time statistics method allows extracting transient dynamic at scales varying from one to few hundreds of nanometers, it predicts the local changes in the number of binding molecules following DSB and can be used to characterize the local dynamic of the chromatin.

Mechanistic model of iodine mass transfer at pool surfaces

Mass transfer of molecular iodine (I2) at the water pool–gas interface can be modeled by means of a water surface film renewal model superimposed to the established two-film theory, where the water-side I2 mass transfer coefficient kw is related to the I2 molecular diffusivity in water D and the air–water contact time a according to The present paper describes a mechanistic approach to determine the contact time from the water flow distribution. The method makes use of a numerical simulation of the poolwater flow, and a numerical evaluation of the contact time distribution at the pool surface. Owing to the numerical treatment it can be applied to pool geometries of any kind, which makes it applicable for nuclear reactor safety studies in general kw=D/πa The present paper describes a mechanistic approach to determine the contact time from the water flow distribution. The method makes use of a numerical simulation of the poolwater flow, and a numerical evaluation of the contact time distribution at the pool surface. Owing to the numerical treatment it can be applied to pool geometries of any kind, which makes it applicable for nuclear reactor safety studies in general.

Simulation of Bed Dynamics and Primary Products from Fast Pyrolysis of Biomass: Steam Compared to Nitrogen as a Fluidizing Agent

Fast pyrolysis of biomass, using steam as a fluidizing agent, provides several benefits. In this paper, an unsteady multiphase computational fluid dynamics (CFD) model coupled with a comprehensive kinetic scheme for primary pyrolysis is used to obtain the formation rates of primary products and compare the profiles when operating with steam and nitrogen. The model only considers the physical effects of the fluidizing gas at the moment, although a literature review indicates the existence of various chemical and surface-interacting effects. At stabilized pyrolysis reaction rates, the product yields were compared to data found in the literature, which indicated similar yields; this supports the correct implementation of the kinetic model. However, the difference in overall rate and composition is very small when steam is compared to nitrogen. The simultaneous simulation of bed dynamics indicate a shifted formation rate of primary products toward the lower part of the fluidized bed, with an increase in solid–vapor contact time and better temperature distribution as a result. More specifically, total heat flux to the biomass increased by 13% in the lowest part of the reactor. In addition, more heat from the sand is carried through the gas phase when using steam: an increase by 9% in the overall reactor (25% in the lowest part), as indicated by the results. Finally, since no substantial differences in overall product formation rate and composition were found, the considerable effect of steam found in experiments and the literature is mainly (not exclusively) attributed to the chemical and surface-interacting mechanisms. Because of the complex nature of secondary pyrolysis in this process, a comprehensive gas-phase kinetic model is needed to investigate the effects of steam further. Coupling of both is difficult, because of computational constraints, as the present model already is very demanding. The obtained profiles of formation rate of primary products can however be used as an input to another model specifically made for studying homogeneous secondary pyrolysis reactions.

Optimisation of the sorption capacity of sulphuric acid activated IBUSA clay in palm oil bleaching using response surface methodology

In this paper, response surface methodology is applied for a detailed optimisation study of the sorption capacity of commercial adsorbent in physical bleaching of palm oil. The study utilised a partial distillation process implemented on a vacuum bleacher using locally sourced IBUSA clay activated with sulphuric acid as the adsorbent medium. The performance of the fractional factorial array in predicting the bleaching capacity of the adsorbent and the interactive sorption effects of the system variables which include; acid concentration, reaction temperature, contact time, adsorbent dosage and particle size distribution are tested within the response surface optimisation algorithm. The result predicts optimum adsorption efficiency of 70.74% at sulphuric acid concentration, reaction temperature, contact time, adsorbent dosage and particle size values of 5.57 mol/l, 122.44°C, 51.73 minutes, 1.78 wt.%, and 347.32 μm respectively. The adequacies of the predictive model and the proposed optimisation method at large are tested using standard statistical criteria and experimental validation.

Experimental process design for sorption capacity of Kogi and Ibusa clay activated with HNO<SUB align="right">3 and H<SUB align="right">2SO<SUB align="right">4 acids in palm oil bleaching

The palm oil bleaching process was optimised through characteristic study of the sorption capacity of Ibusa and Kogi clay activated with nitric and sulphuric acids. The raw clays were characterised for structure elucidation via FTIR and X-ray fluorescence spectroscopic techniques and the result indicates that Kogi clay is predominantly kaolinite while Ibusa sample is montmorillonite. The response surface methodology based on 42-full factorial design was applied for the process analysis. Effects of process variables such as acid concentration, bleaching temperature, contact time, clay-oil ratio and particle size distribution was analysed employing the characterisation-control-optimisation paradigm necessary for detailed process design for optimum bleaching efficiency. A sequence of experiments was carried out at different settings of the process variables and their interactive sorption effects were calculated in terms of adsorption efficiency. Acid concentration and bleaching time were found to be the most significant process parameter. Predictive equations describing the optimum bleaching efficiency in terms of the process variables were derived from regression analysis of the experimental data. A maximum mean adsorption strength of 40 units was predicted for both specimens through activation wit HNO3 acid at mean acid concentration of 6.12 mol/l and mean bleaching time of 48.88 mins, while H2SO4 acid recorded relatively low adsorption strength of 24 units with Kogi clay and negligible strength of 8 units with Ibusa sample at the same experimental condition. Overall, HNO3 acid activated Kogi clay shows robustness to expanded variable effects and could be recommended for general application in lieu of bentonite.

Interfacial Protein–Protein Associations

While traditional models of protein adsorption focus primarily on direct protein-surface interactions, recent findings suggest that protein-protein interactions may play a central role. Using high-throughput intermolecular resonance energy transfer (RET) tracking, we directly observed dynamic, protein-protein associations of bovine serum albumin on polyethylene glycol modified surfaces. The associations were heterogeneous and reversible, and associating molecules resided on the surface for longer times. The appearance of three distinct RET states suggested a spatially heterogeneous surface - with areas of high protein density (i.e., strongly interacting clusters) coexisting with mobile monomers. Distinct association states exhibited characteristic behavior, i.e., partial-RET (monomer-monomer) associations were shorter-lived than complete-RET (protein-cluster) associations. While the fractional surface area covered by regions with high protein density (i.e., clusters) increased with increasing concentration, the distribution of contact times between monomers and clusters was independent of solution concentration, suggesting that associations were a local phenomenon, and independent of the global surface coverage.

Ozone inactivation of resistant microorganisms: Laboratory analysis and evaluation of the efficiency of plants

In this work, the ozone inactivation of resistant microorganisms is studied and a method to assess the efficiency of a drinking water plant to inactivate resistant microorganisms using ozone is proposed. This method aims at computing the fraction of resistant microorganisms that are not inactivated at the exit of an ozonation step by evaluating the duration of the lag phase of the ozone inactivation of these microorganisms and the contact time distribution of these microorganisms with the ozone in the step. To evaluate the duration of the lag phase of the ozone inactivation of resistant pathogenic microorganisms, an experimental procedure is proposed and applied to Bacillus subtilis spores. The procedure aims at characterizing the ozone inactivation kinetics of B. subtilis spores for different temperature and ozone concentration conditions. From experimental data, a model of the ozone inactivation of B. subtilis spores is built. One of the parameters of this model is called the lag time and it measures the duration of the lag phase of the ozone inactivation of B. subtilis spores. This lag time is identified for different temperature and ozone concentration conditions in order to establish a correlation between this lag time and the temperature and ozone concentration conditions. To evaluate the contact time distribution between microorganisms and the ozone in a disinfection step of a drinking water plant, a computational fluid dynamics tool is used. The proposed method is applied to the ozonation channel of an existing drinking water plant located in Belgium and operated by Vivaqua. Results show that lag times and contact times are both in the same order of magnitude of a few minutes. For a large range of temperatures and ozone concentrations in the Tailfer ozonation channel and for the highest hydraulic flow rate applied, a significant fraction of resistant microorganisms similar to B. subtilis spores is not inactivated.

An Efficient Prediction-Based Routing in Disruption-Tolerant Networks

Routing is one of the most challenging, open problems in disruption-tolerant networks (DTNs) because of the short-lived wireless connectivity environment. To deal with this issue, researchers have investigated routing based on the prediction of future contacts, taking advantage of nodes' mobility history. However, most of the previous work focused on the prediction of whether two nodes would have a contact, without considering the time of the contact. This paper proposes predict and relay (PER), an efficient routing algorithm for DTNs, where nodes determine the probability distribution of future contact times and choose a proper next-hop in order to improve the end-to-end delivery probability. The algorithm is based on two observations: one is that nodes usually move around a set of well-visited landmark points instead of moving randomly; the other is that node mobility behavior is semi-deterministic and could be predicted once there is sufficient mobility history information. Specifically, our approach employs a time-homogeneous semi-Markov process model that describes node mobility as transitions between landmarks. Then, we extend it to handle the scenario where we consider the transition time between two landmarks. A simulation study shows that this approach improves the delivery ratio and also reduces the delivery latency compared to traditional DTN routing schemes.

Collision of polymers in a vacuum

In a number of experimental situations, single-polymer molecules can be suspended in a vacuum. Here collisions between such molecules are considered. The limit of high collision velocity is investigated numerically for a variety of conditions. The distribution of contact times, scattering angles, and final velocities are analyzed. In this limit, self-avoiding chains are found to become highly stretched as they collide with each other and have a distribution of scattering times that depends on the scattering angle. The velocity of the molecules after the collisions is similar to predictions of a model assuming thermal equilibration of molecules during the collision. The most important difference is a significant subset of molecules that inelastically scatter but do not substantially change direction.

A multi‐scale theoretical model for gas–Liquid interface mass transfer based on the wide spectrum eddy contact concept

AbstractOn the basis of the wide spectrum eddy contact concept and the isotropic turbulence theory, a multi‐scale theoretical model for the prediction of liquid‐side mass transfer coefficient in gas–liquid system was developed. The model was derived from an unsteady‐state convection and diffusion equation and considered the contributions of eddies with different sizes to the overall mass transfer coefficient. The proper contact time distribution at the surface is need to be determined to obtain satisfactory results with this model. Moreover, a simplified model was also proposed based on the assumption of steady‐state mass transfer mechanism for single eddy. The results predicted by this model showed a very good agreement with the available experimental data in a comparatively wide range of turbulence intensities. © 2010 American Institute of Chemical Engineers AIChE J, 2011

Contact time in random walk and random waypoint: Dichotomy in tail distribution

Contact time (or link duration) is a fundamental factor that affects performance in Mobile Ad Hoc Networks. Previous research on theoretical analysis of contact time distribution for random walk models (RW) assume that the contact events can be modeled as either consecutive random walks or direct traversals, which are two extreme cases of random walk, thus with two different conclusions. In this paper we conduct a comprehensive research on this topic in the hope of bridging the gap between the two extremes. The conclusions from the two extreme cases will result in a power-law or exponential tail in the contact time distribution, respectively. However, we show that the actual distribution will vary between the two extremes: a power-law-sub-exponential dichotomy, whose transition point depends on the average flight duration. Through simulation results we show that such conclusion also applies to random waypoint.

Probabilistic Approach to Particle-Wall Contact Time in Fluidized Beds

The heat transfer between submerged surface and particles in fluidized bed is affected by particle migration to and from the exchanger surface. A new probabilistic model was developed for particle migration and particle-wall contact time distribution based on a classical gamma function. The existing models suggest a decrease in contact time by increasing the gas velocity. However, it has been experimentally shown that the contact time increases in turbulent regime by increasing the gas velocity. A theoretical probabilistic model was developed to represent such a trend. The model is in good agreement with the experimental data.

Force‐ and length‐dependent catastrophe activities explain interphase microtubule organization in fission yeast

The cytoskeleton is essential for the maintenance of cell morphology in eukaryotes. In fission yeast, for example, polarized growth sites are organized by actin, whereas microtubules (MTs) acting upstream control where growth occurs. Growth is limited to the cell poles when MTs undergo catastrophes there and not elsewhere on the cortex. Here, we report that the modulation of MT dynamics by forces as observed in vitro can quantitatively explain the localization of MT catastrophes in Schizosaccharomyces pombe. However, we found that it is necessary to add length-dependent catastrophe rates to make the model fully consistent with other previously measured traits of MTs. We explain the measured statistical distribution of MT–cortex contact times and re-examine the curling behavior of MTs in unbranched straight tea1Δ cells. Importantly, the model demonstrates that MTs together with associated proteins such as depolymerizing kinesins are, in principle, sufficient to mark the cell poles.

Relationship between interparticle contact lifetimes and rheology in gravity-driven granular flows

The validity of the Bagnold constitutive relation in gravity-driven granular flow down an inclined plane is studied by discrete element (DEM) simulations. In the limit of infinitely hard particles, the Bagnold relation is known to hold exactly. We determine deviations from this relation as a function of all parameters governing interparticle interactions. These include elastic compliance, inelastic dissipation, friction coefficient, and interparticle cohesion. We find significant deviations from Bagnold rheology in some regions of this parameter space and propose a generalized Bagnold relation to account for this effect. Moreover, we note a significant correlation between the breakdown of Bagnold rheology in the bulk and the appearance of a long-time tail in the two-particle contact time distributions.

Impact of different recirculation schemes on nitrogen removal and overall performance of a laboratory scale MBR

For membrane bioreactors (MBR) with enhanced nutrients removal, rather complex recirculation schemes based on the biological requirements are commonly recommended. The aim of this work was to evaluate other recirculation options. For a laboratory scale MBR, four different recirculation schemes were tested. The MBR was operated with COD degradation, nitrification, post-denitrification without carbon dosing and biological phosphorus removal. For all configurations, efficient COD, nitrogen and phosphorus removal could be achieved. There were no big differences in elimination efficiency between the configurations (COD elimination: 96.6-97.9%, nitrogen removal: 89.7-92.1% and phosphorus removal: 97.4-99.4%). Changes in the degradation, release and uptake rates were levelled out by the changes in contact time and biomass distribution. With relatively constant outflow concentrations, different configurations are still interesting with regard to oxygen consumption, simplicity of plant operation or support of certain degradation pathways such as biological phosphorus removal or denitrification.

Droplet–particle collision mechanics with film-boiling evaporation

A three-dimensional numerical model is developed to simulate the process of collision between an evaporative droplet and a high-temperature particle. This phenomenon is of direct relevance to many engineering process operations, such as fluid catalytic cracking (FCC), polyethylene synthesis, and electronic materials coating. In this study, the level-set method and the immersed-boundary method are combined to describe the droplet–particle contact dynamics in a fixed Eulerian grid. The droplet deformation is captured by one level-set function while the solid–fluid boundary condition is imposed on the particle surface through the immersed-boundary method involving another level-set function. A two-dimensional vapour-layer model is developed to simulate the vapour flow dynamics. Equations for the heat transfer characteristics are formulated for each of the solid, liquid and gas phases. The incompressible flow-governing equations are solved using the finite-volume method with the ALE (arbitrary Lagrangian Eulerian) technique. The simulation results are validated through comparisons with experimental data obtained from the new experimental set-up designed in this study. An important feature of the droplet impacting on a particle with film boiling is that the droplet undergoes a spreading, recoiling and rebounding process, which is reproduced by the numerical simulation based on the model. Details of the collision such as spread factor, contact time and temperature distribution are provided. Simulations are also conducted to examine the effects of the particle size and the collision velocity. Although the value for the maximum spread factor is larger for a higher impact velocity and for a smaller particle, the contact time is independent of the impact velocity and particle size. Both the normal collision and the oblique collision are considered in this study.

Flow field investigation in a photocatalytic reactor for air treatment (Photo-CREC–air)

The aerodynamic behavior of a photocatalytic reactor for air treatment, Photo-CREC–air, with demonstrated high quantum efficiency performance, is examined using CFX-5.7.1. Photo-CREC–air consists of a venturi section that features low pressure drop and uniform illumination of the photocatalyst, resulting in high oxidation quantum efficiencies. The numerical simulations allowed the identification of several design issues in the original Photo-CREC–air unit, which include extensive boundary layer separation close to the photocatalyst support and regions of flow recirculation that render ca. 77% of the support surface area inactive. The simulations reveal that this issue could be addressed by replacing the wire-mesh basket sidewalls with perforated plates. This modification causes an increase in the pressure drop downstream of the support and achieves significant uniformization of the mass flow and air–photocatalyst contact time distributions. A modified Photo-CREC–air design is also presented and studied using CFX-5.7.1. This modified design is envisaged with the objective of improving UV-irradiation uniformity, an issue that is not completely addressed in the original design due to the shape of the windows and divergent section. CFD simulations reveal that, although the flow field is uniform, mass flow and contact time distributions are not. Nonetheless, this problem is addressed by increasing the pressure drop downstream of the support through the addition of a region modeled as a perforated plate. The simulations reveal that the mass flow and contact time distributions are significantly uniformized once this modification is implemented.