Residence Time Measurements Research Articles

Cylindrical object contact detection for use in discrete element method simulations, Part II—Experimental validation

This article is the second of two focusing on cylindrical object contact detection. The first part presents algorithms for determining if contact occurs between a cylindrical object and a plane, and between two cylindrical objects, in addition to expressions for contact overlap, contact normal vector, and contact location. This article presents the tests and results used to quantitatively validate the algorithms implemented in three-dimensional discrete element method (DEM) simulations. In particular, four tests are performed. The first consists of a comparison against analytical expressions for post-collision translational and rotational velocities for a single cylinder impacting a flat wall. The second involves comparisons against experimental residence time measurements of a cylindrical particle moving within a baffled, rotating horizontal cylinder. The third test consists of comparisons against experimental bulk density measurements of cylindrical particles dropped into a cylindrical container. The last test compares experimental and computational values of the dynamic angle of repose in a rotating drum. In addition to true cylindrical objects, comparisons are also made in the first three tests against two cylindrical object approximations using glued spheres. The true cylinder model performs well in each of the tests while the glued sphere approximations perform poorly, especially when fewer spheres are used to represent the cylinder shape. These studies demonstrate that the cylinder contact algorithms presented in the first article perform correctly, and more accurately model real cylinder behavior than glued sphere approximations.

Drug–target residence time: critical information for lead optimization

Failure due to poor in vivo efficacy is a primary contributor to attrition during the development of new chemotherapeutics. Lead optimization programs that in their quest for efficacy focus solely on improving the affinity of drug-target binding are flawed, since this approach ignores the fluctuations in drug concentration that occur in vivo. Instead the lifetime of the drug-target complex must also be considered, since drugs only act when they are bound to their targets. Consequently, to improve the correlation between the in vitro and in vivo activity of drugs, measurements of drug-target residence time must be incorporated into the drug discovery process.

Flow Control in Six-Strand Billet Continuous Casting Tundish With Different Configurations

A1:2.5 scale tundish model was set up in laboratory (or a six-strand billet continuous casting tundish with different configurations to investigate fluid flow characteristics under different operational conditions by measuring residence time distribution curves. It was found that minimum residence time, maximum concentration time and average residence time of the three strands on the same side of the tundish with the former configuration under normal operation, that is, six strands were open, were small and non-uniform and the tundish had large dead volume fraction. Vortexes easily formed on the liquid surface in the pouring zone of the tundish. The fluid flow characteristics in the tundish with the optimal turbulence inhibitor and baffles were improved and became less non-uniform among the strands. Vortexes were not found on the pouring zone surface in the optimal tundish. For non-normal operation, that is, one strand was close, it was important to choose which strand to be closed for maintaining flow characteristics of the rest two strands. It was found from this investigation that fluid flow characteristics in the optimal configuration tundish with closing strand 2 were better than those with closing strand 3 on the same side.

Prediction of segregation tendency in dry particulate pharmaceutical mixtures: Application of an adapted mathematical tool to cohesive and non-cohesive mixtures

The measurement of average residence times and their variance, used to calculate the deviation of chemical reactors from the ideal behaviour of a perfectly-mixed vessel, or a plug flow pattern, has already been proposed in the literature to evaluate the segregation tendency of granular mixtures. The method consists of introducing pulse perturbation (of another material) to the established regular flow of a single granular material or a granular mixture and to assess the response of the system in terms of pulsed material concentration at the process outlet. The particles’ average residence time and its standard deviation are then related to segregation tendency. Results from the application of this new method are useful when compared to those obtained from a reference mixture to be chosen according to a particular formulation development or process understanding need. This work applies the proposed method for various mixtures, both cohesive and non-cohesive, and derives phenomenological mathematical models expressing segregation tendency as a function of the parameters shown to be critical (i.e. statistically significant) to component segregation. The models were shown to be statistically and experimentally robust in the case of non-cohesive to slightly cohesive mixtures. Although the mathematical models are phenomenological, the findings allow for deriving mechanistic explanations on segregation tendency.

Planting intensity, residence time, and species traits determine invasion success of alien woody species

We studied the relative importance of residence time, propagule pressure, and species traits in three stages of invasion of alien woody plants cultivated for about 150 years in the Czech Republic, Central Europe. The probability of escape from cultivation, naturalization, and invasion was assessed using classification trees. We compared 109 escaped-not-escaped congeneric pairs, 44 naturalized-not-naturalized, and 17 invasive-not-invasive congeneric pairs. We used the following predictors of the above probabilities: date of introduction to the target region as a measure of residence time; intensity of planting in the target area as a proxy for propagule pressure; the area of origin; and 21 species-specific biological and ecological traits. The misclassification rates of the naturalization and invasion model were low, at 19.3% and 11.8%, respectively, indicating that the variables used included the major determinants of these processes. The probability of escape increased with residence time in the Czech Republic, whereas the probability of naturalization increased with the residence time in Europe. This indicates that some species were already adapted to local conditions when introduced to the Czech Republic. Apart from residence time, the probability of escape depends on planting intensity (propagule pressure), and that of naturalization on the area of origin and fruit size; it is lower for species from Asia and those with small fruits. The probability of invasion is determined by a long residence time and the ability to tolerate low temperatures. These results indicate that a simple suite of factors determines, with a high probability, the invasion success of alien woody plants, and that the relative role of biological traits and other factors is stage dependent. High levels of propagule pressure as a result of planting lead to woody species eventually escaping from cultivation, regardless of biological traits. However, the biological traits play a role in later stages of invasion.

Modeling of protein breakthrough performance in cryogel columns by taking into account the overall axial dispersion

A model considering the overall axial dispersion for describing protein adsorption and breakthrough in monolithic cryogel beds has been developed. The microstructure of cryogels was characterized by tortuous capillaries with a normal diameter distribution but a constant pore wall thickness. The axial dispersion within cryogel columns was described by using the overall axial dispersion coefficient, which can be easily obtained by matching the experimental breakthrough curves without adsorption or measuring residence time distributions (RTDs). Experimental breakthrough curves of lysozyme within a metal-chelated affinity cryogel by Persson et al. (Biotechnol. Bioeng. 2004, 88, 224-236) and a cation-exchange cryogel by Yao et al. (J. Chromatogr. A 2007, 1157, 246-251) were employed as examples to test the model. The results showed that by using the axial dispersion coefficient and assuming uniform radial concentration profile at a given cross-section of the cryogel along the bed height, the model can describe the detailed behaviors of the in-bed overall axial dispersion, the in-pore mass transfer, as well as the protein adsorption and breakthrough. For a known overall axial dispersion coefficient, the lumped parameter of the mass transfer coefficient between the bulk liquid and the capillary wall can be determined by fitting the protein breakthrough curve at a known chromatographic condition. Once this parameter is determined, the model can be used to predict the protein breakthrough profiles under different conditions based on the basic physical parameters of the cryogel bed and the properties of the fluid and protein. The effective capillary diameters employed in the model are close to the actual pore sizes observed from the images by SEM. The model predictions of lysozyme breakthrough profiles at various flow rates are also in good agreement with the experimental data in both the metal-chelated affinity and cation-exchange cryogel columns.

Prediction of segregation tendency in dry particulate pharmaceutical mixtures: Application of an adapted mathematical tool to cohesive and non-cohesive mixtures

The measurement of average residence times and their variance, used to calculate the deviation of chemical reactors from the ideal behaviour of a perfectly-mixed vessel, or a plug flow pattern, has already been proposed in the literature to evaluate the segregation tendency of granular mixtures. The method consists of introducing pulse perturbation (of another material) to the established regular flow of a single granular material or a granular mixture and to assess the response of the system in terms of pulsed material concentration at the process outlet. The particles' average residence time and its standard deviation are then related to segregation tendency. Results from the application of this new method are useful when compared to those obtained from a reference mixture to be chosen according to a particular formulation development or process understanding need. This work applies the proposed method for various mixtures, both cohesive and non-cohesive, and derives phenomenological mathematical models expressing segregation tendency as a function of the parameters shown to be critical (i.e. statistically significant) to component segregation. The models were shown to be statistically and experimentally robust in the case of non-cohesive to slightly cohesive mixtures. Although the mathematical models are phenomenological, the findings allow for deriving mechanistic explanations on segregation tendency.

Measurement of Residence Time in Hot Compressed Water: 2. Salts in Continuously Stirred Tank Reactors

AbstractThe behavior of alkali salts in a continuously stirred tank reactor is of particular interest with respect to the development of the hydrothermal biomass gasification process. On the one hand, alkali salts increase gas formation. On the other hand, they may plug reactors due to precipitation. To obtain information on the potential enrichment of salts in continuously stirred tank reactors, cumulative residence time distribution functions were measured using K2CO3 and NaHCO3 tracers and phenol as a reference.

Numerical Simulation of Blood Flow Through Microvascular Capillary Networks

A numerical method is implemented for computing blood flow through a branching microvascular capillary network. The simulations follow the motion of individual red blood cells as they enter the network from an arterial entrance point with a specified tube hematocrit, while simultaneously updating the nodal capillary pressures. Poiseuille's law is used to describe flow in the capillary segments with an effective viscosity that depends on the number of cells residing inside each segment. The relative apparent viscosity is available from previous computational studies of individual red blood cell motion. Simulations are performed for a tree-like capillary network consisting of bifurcating segments. The results reveal that the probability of directional cell motion at a bifurcation (phase separation) may have an important effect on the statistical measures of the cell residence time and scattering of the tube hematocrit across the network. Blood cells act as regulators of the flow rate through the network branches by increasing the effective viscosity when the flow rate is high and decreasing the effective viscosity when the flow rate is low. Comparison with simulations based on conventional models of blood flow regarded as a continuum indicates that the latter underestimates the variance of the hematocrit across the vascular tree.

The use of momentum ratio to evaluate the performance of CSTRs

In this study, the residence time distribution of a CSTR was measured and analysed for variations in CSTR feed geometry, impeller speed and inlet flow rate. The measured residence time distributions were also used to characterise the degree of plug flow behaviour and short-circuiting. The data showed that the performance of a CSTR could not be evaluated using the ratio of the mean residence time to the batch-mode mixing time ( τ/ t M ), and that this approach can lead to process over-design and excessive energy requirements. Instead, it was shown that all process parameters could be correlated using the ratio of the inlet jet momentum to the impeller discharge momentum. The prediction of the degree of short-circuiting and plug flow in the CSTR could be used to improve process control. In addition, the results can be used to identify whether the inlet position or diameter of an existing CSTR should be modified to improve process performance.

Structure Optimization of Horizontal Continuous Casting Tundishes Using Mathematical Modeling and Water Modeling

The configuration of a tundish for a two-strand horizontal continuous caster was designed and optimized using water modeling, mathematical modeling and industrial trials. Five designs were studied: the original tundish without flow control devices, the tundish with a turbulence inhibitor at the bottom, the tundish with a deep inlet launder and a tilted dam at the end of the inlet launder, the tundish with two dams with holes, and the tundish with a shallow inlet launder and a high dam in the main chamber. Water modeling was used to measure residence time and investigate dead zone fractions and fluid flow patterns. In the mathematical modeling, fluid flow, heat transfer and inclusion motion and removal were calculated. In industrial trials, the total oxygen, nitrogen pick-up, and inclusions in steel samples taken from the tundish and the billets were analyzed. The results indicated that the tundish with an inlet launder and a dam either at the end of the chamber or in the main chamber was the best design.

Biological wastewater treatment in the inverse fluidised bed reactor

Abstract The biological wastewater treatment was investigated in the inverse fluidised bed reactor (IFBR) in which polypropylene particles of density 910 kg/m 3 were fluidised by an upward flow of gas. Measurements of chemical oxygen demand (COD) versus residence time t were performed for various ratios of settled bed volume to reactor volume ( V b / V R ) and air velocities u g . The largest COD removal was attained when the reactor was operated at the ratio ( V b / V R ) m = 0.55 and an air velocity u gm = 0.024 m/s. Under these conditions, the value of COD was practically at steady state for times greater than 30 h. Thus, these values of ( V b / V R ) m , u gm and t can be considered as the optimal operating parameters for a reactor when used in treatment of industrial wastewaters. A decrease in COD from 36,650 to 1950 mg/l, i.e. a 95% COD reduction, was achieved when the reactor was optimally controlled at ( V b / V R ) m = 0.55, u gm = 0.024 m/s and t = 30 h. The pH was controlled in the range 6.5–7.0 and the temperature was maintained at 28–30 °C. The biomass loading was successfully controlled in an IFBR with support particles whose matrix particle density was smaller than that of liquid. The steady-state biomass loading depended on the ratio ( V b / V R ) and an air velocity u g . In the culture conducted after switching from the batch to the continuous operation, the steady-state biomass loading was attained after approximately 2-week operation. In the cultures conducted after change in ( V b / V R ) at a set u g , the steady-state mass of cells grown on the particles was achieved after about 6-day operation. For a set ratio ( V b / V R ), the biomass loading depended on u g . With change in u g at a set ( V b / V R ), the new steady-state biomass loading occurred after the culturing for about 2 days.

Allosteric Effects on Substrate Dissociation from Cytochrome P450 3A4 in Nanodiscs Observed by Ensemble and Single-Molecule Fluorescence Spectroscopy

Cytochrome P450 (CYP) 3A4 is a major human drug-metabolizing enzyme and displays pharmacologically relevant allosteric kinetics caused by multiple substrate and/or effector binding. Here, in the first single-molecule (SM) fluorescence studies of CYPs, we use total internal reflection fluorescence microscopy to measure residence times of the fluorescent dye Nile Red in CYP3A4 incorporated in surface-immobilized lipid Nanodiscs, with and without the effector alpha-naphthoflavone. We find direct evidence that CYP3A4 effectors can decrease substrate off-rates, providing a possible mechanism for effector-mediated enhancement of substrate metabolism. These interesting results highlight the potential of SM methods in studies of CYP allosteric mechanisms.

Froth mean residence time measurement in industrial flotation cells

Froth plays an important role in flotation processes preventing the pulp transport to the concentrate (short-circuit). Thus, it contributes to increasing the concentrate grade by gravity drainage of entrained particles, back into the pulp. Key parameters affecting the froth performance are the mean residence times of solids, liquid and gas in the froth. The froth mean residence time depends on the froth depth, gas flowrate, gas hold-up, and flow regime. In this work, the froth mean residence times were evaluated from direct measurements of liquid and solid time responses in the froth of self-aerated copper flotation cells of 130 m 3. For this purpose the radioactive tracer technique was applied, using 82Br as liquid tracer, and non-floatable mineral particles in three size classes (+150; −150 + 45;−45 μm) as solid tracers. All tracers were injected at the cell feed entrance, which allowed the tracer to circulate first through the rotor, and become well distributed over the whole cross-sectional area before entering the froth. Each tracer time response was measured on-line below the pulp/froth interface (input signal) and at the concentrate overflow discharge (output signal). The froth mean residence time was then obtained by difference between the average times of the froth input and output tracer signals, previously modelled. For the copper rougher flotation, the froth mean residence time (9–12 s) of non-floatable solids, derived from experimental measurement, was comparable with that obtained by measuring the gas flowrate and estimating the effective gas volume in the froth. While, the froth mean residence times of liquid and floatable solid were significantly larger, 21 and 24 s, respectively.

Measurement of gas phase characteristics using a monofibre optical probe in a three-phase flow

The study of gas–liquid–solid systems structure requires reliable measurement tools. In this paper, preliminary results on the potential use of a monofibre optical probe to investigate such flow are presented. This probe, manufactured at LEGI, allows the simultaneous measurement of the gas phase residence time and gas phase velocity. This specificity makes this probe more interesting than classical single tip probes (which measure only the gas residence time) or double tip probes (which are more intrusive). Although extensively used in two-phase gas–liquid, this probe was never used in gas–liquid–solid systems. First, the probe signal response is studied for three-phase flow conditions in the presence of solids. Results show that for soft solids, the probe tips can be contaminated when the probe pierces the solid. The signal processing procedure was modified accordingly to take into account these events. Second, the probe results are validated by comparing global results (global void fraction, gas flowrate) deduced from profile measurements with measurements performed by independent means. Lastly, void fraction profiles and interfacial area are studied more in detail. Depending on the solid loading, these profiles exhibit different behaviours. These features are associated to characteristics of the flow such as the transition from an homogeneous regime to an heterogenous regime, and are consistent with global observation performed by independent means. This demonstrates the ability of the probe to connect local information to the global behaviour and structure of the flow.

The influence of a cone-in-cone insert on flow pattern and wall pressure in a full-scale silo

A silo insert is sometimes placed inside the hopper to improve the flow pattern of solids in a funnel flow silo. This paper describes a recent experimental investigation of the flow pattern and wall pressures observed during filling and emptying of a cylindrical silo with and without a cone-in-cone insert. The 2.5 m diameter silo had an 8 m high cylindrical section and a 44° conical hopper, and was filled with crushed quartz sand. The wall pressures were measured at ten chosen locations using pressure cells mounted on the wall. To investigate the solids flow pattern, markers were placed at fixed positions in the silo during filling and the residence time for each marker during discharge was observed. The flow pattern can be inferred from the residence time measurements for the markers and the surface profile observations during discharge. These tests show that a cone-in-cone insert can have a strong influence on both the flow pattern during discharge and the pressure acting on the silo wall and its industrial use should be considered cautiously and implemented with care.

TRIPHASIC EXTERNAL-LOOP AIRLIFT REACTORS. HYDRODYNAMIC AND DISPERSION STUDIES

Considering that three-phase dispersions are common in chemical engineering and biotechnology, and with increasing airlift reactor applications to chemical and biotechnological processes, a experimental study was undertaken in order to investigate the liquid phase hydrodynamics and dispersion in an external-loop airlift reactor on laboratory scale. Experiments were conducted in order to find a relationship between relevant hydrodynamic and mixing parameters of a solid-suspended external-loop airlift reactor of laboratory scale, and operating variables such as gas velocity, solids loading and solids density, which should be the basis for extending the study to larger scale contactors. It was found that these parameters affect the hydrodynamic characteristics of the investigated external-loop airlift reactor and empirical correlations for gas holdup and liquid circulation velocity were obtained as functions on the above-mentioned operating variables. The lateral and axial mixing of dispersed particles in a circulating fluidized bed were measured by using the phosphor tracer technique. The dispersion model satisfactorily described the measured residence time distribution.

Modelling of Inclusion Motion and Flow Patterns in Swirling Flow Tundishes with Symmetrical and Asymmetrical Structures

Swirling flow tundish (SFT) has recently been developed as a new type of tundish. Its key mechanism is forming swirling flow to enhance inclusion removal from steel for high quality slab casting. This is achieved by guiding the liquid steel into the tundish through the tangential inlet of a cylindrical swirling chamber (SC). This can restrain and alleviate the turbulence of the impact zone. SC has been settled into two types of tundishes-originally symmetrical and asymmetrical. To understand the fundamentals, physical and mathematical modellings of this new process have been conducted in this work. Physical modellings are carried out with the utilization of an asymmetrical one-strand 1 : 2.5 and a symmetrical two-strand 1 : 3 scale models. The internal flow is measured by digital particle image velocimetry (DPIV). Numerical modelling is carried out in line with the physical modelling to examine the details of the flow patterns and rotational effect caused by the SC. The trajectories of non-metallic inclusion particles are also studied by discrete phase model (DPM) within the commercial CFD package environment, FLUENT. The mathematical model proposed is validated by comparing the predicted and measured residence time distribution and velocity fields. The results show that both originally symmetrical and asymmetrical flow fields have been changed to asymmetrical ones after setting SC into it. And the results show that the flow is slower behind the dam and weir. Examinations of the trajectories of inclusion particles suggest that the inclusion removal capacity of the SFT is higher than the tundish with a turbulence inhibitor (TI). From the separation ratio results, it can be seen that SFT has higher ability to remove small inclusion particles than a tundish with TI, which is of great significance especially for production of high-quality steel.

Changes in dissolved silicate loads to the Baltic Sea — The effects of lakes and reservoirs

We tested the hypothesis that dissolved silicate (DSi) yields [kg km − 2 yr − 1 ] of 82 major watersheds of the Baltic Sea can be expressed as a function of the hydraulic load (HL) as a measure of water residence time and the total organic carbon (TOC) concentration, both variables potentially increasing the DSi yield. Most boreal rivers fitted a linear regression model using HL as an independent variable to explain the DSi yield. Rivers with high HL, i.e., shortest residence times, showed highest DSi yields up to 2300 kg km − 2 yr − 1 . This is most likely caused by an excess supply of DSi, i.e., the geochemical sources prevail over biological sinks in these boreal watersheds. The DSi yield for regulated and unregulated larger rivers of the boreal watersheds constituting about 40% of the total water discharge and of the total DSi load to the Baltic Sea, respectively, can be expressed as: DSi yield = 190 + 49.5 HL[m yr − 1 ] + 0.346 TOC [µM] ( R 2 = 0.80). Since both HL and TOC concentrations have decreased after damming, the DSi yields have decreased significantly in the regulated boreal watersheds, for the River Luleälven we estimated more than 30%. The larger eutrophic watersheds draining cultivated landscape of the southern catchment of the Baltic Sea and representing about 50% of the annual water discharge to the Baltic Sea, deviated from this pattern and showed lower DSi yields between 60–580 kg km − 2 yr − 1 . DSi yields showed saturation curve like relationship to HL and it appears that DSi is retained in the watersheds efficiently through biogenic silica (BSi) production and subsequent sedimentation along the entire river network. The relationship between HL and DSi yields for all larger cultivated watersheds was best fitted by a Freundlich isotherm (DSi = 115.7HL 109; R 2 = 0.73), because once lake and reservoir area exceeds 10% of the watershed area, minimum DSi yields were reached. To estimate an uperturbed DSi yield for the larger eutrophic southeastern watersheds is still difficult, since no unperturbed watersheds for comparison were available. However, a rough estimate indicate that the DSi flux from the cultivated watersheds to the Baltic Sea is nowadays only half the uperturbed flux. Overall, the riverine DSi loads to the Baltic Sea might have dropped with 30–40% during the last century.

Fluorophotometric measurement of the precorneal residence time of topically applied hyaluronic acid

Purpose:This study was performed to separately assess the aqueous flow applied with hyaluronic acid, and the behaviour of hyaluronic acid itself on the ocular surface.Methods:Two different fluorescent dyes, fluorescein sodium...