Tracer Pulse Method Research Articles

CFD Modeling and Simulation of the Axial Dispersion Characteristics of a Fixed-Bed Reactor.

In this study, the axial dispersion characteristics of a fixed-bed reactor with different packed structures were investigated via computational fluid dynamics (CFD) simulation. The discrete element method was employed to develop the physical model of a fixed bed. Then, CFD simulations were performed to investigate the flow resistance coefficient under different Reynolds numbers. The prediction values were in fair agreement with those calculated by the Carman equation, thereby validating the proposed CFD model. The tracer pulse method and the step method were employed to evaluate the residence time distribution characteristics in the fixed-bed reactors where the mean residence time and axial dispersion coefficient were calculated. The distribution characteristics of the tracer concentration and fluid velocity were also obtained and used to explain the mixing performance of the fixed bed. This simulation study can contribute to the optimization design and scaling up of reactors with porous packed structures.

Evaluation of the mixing quality of high-viscosity yield stress fluids in a tubular reactor

Abstract In this study, the mixing quality of high-viscosity yield stress fluid (Carbopol aqueous solution) under laminar and turbulent flow regimes was evaluated through a numerical experimental study. A three-dimensional computational fluid dynamics large-eddy simulation (CFD-LES) model was employed to capture large-scale vortex structures. The proposed CFD model was validated by the experimental data in terms of mean velocity profiles and velocity-time history. Thereafter, the CFD model was applied to simulate the residence time distribution using the tracking technique: tracer pulse method and step method. In addition, the non-ideal flow phenomena caused by molecular diffusion and eddy diffusion were evaluated. The effects of the rheological properties on the mixing performance were also investigated. The presented results can provide useful guidance to enhance mass transfer in reactors with high-viscosity fluids.

A method for determining steady velocity of shallow water flow on hill-slope and the distance when water flow reaches stability

Understanding the velocity distribution of shallow water flow along hill-slope is of great significance in soil erosion studies. This study proposes a method to estimate the steady flow velocity and the distance when water flow reaches stability, using hill-slope flow velocity distribution data along a simulated rill measured by the electrolyte tracer pulse method. Laboratory experiments were performed using a flume of 12 m long, 0.1 m wide and 0.3 m height under five slope gradients (5°, 10°, 15°, 20° and 25°) and four flow discharges (2, 4, 8 and 16 L min−1). The electrolyte tracer pulse method was employed to measure the flow velocities at locations of 1, 2, 3, 4, 6, 8, 10 and 12 m from the inlet of the flowing water. The flow velocities measured by the dye tracer method served as a control. The results showed that the flow velocities measured by the electrolyte tracer pulse method initially accelerated and then reached a steady value. An equation for estimating flow velocity on the basis of the measurement distance was established. The flow velocities calculated by the equation agreed well with those measured by the electrolyte tracer pulse method. The determination (R2) and the Nash-Sutcliffe model efficiency (NSE) were greater than 0.800, except for a few cases. In addition, the equation established in this study was shown to successfully predict flow velocities measured in other studies. The steady flow velocities and the distances when water flow reached stability under different slope gradients and flow discharges were determined by this equation. When the differences between the measured flow velocity and the steady flow velocity were very small, that is, 5% and 10%, the steady distances ranged from 2.239 m to 4.772 m and from 2.843 m to 6.059 m, respectively. By comparing the steady flow velocities with the flow velocities measured by the dye tracer method, the results indicated that the steady flow velocities were 0.702–0.735 times that of the flow velocities measured by the dye tracer method at various slope gradients and flow discharges. A linear function existed between the two, and the regressed parameter k (i.e., 0.718) could be used as the correction factor between the two. In general, the equation established in this study can facilitate the prediction of the steady flow velocity.

A closer study of methanol adsorption and its impact on solute retentions in supercritical fluid chromatography

Surface excess adsorption isotherms of methanol on a diol silica adsorbent were measured in supercritical fluid chromatography (SFC) using a mixture of methanol and carbon dioxide as mobile phase. The tracer pulse method was used with deuterium labeled methanol as solute and the tracer peaks were detected using APCI-MS over the whole composition range from neat carbon dioxide to neat methanol. The results indicate that a monolayer (4Å) of methanol is formed on the stationary phase. Moreover, the importance of using the set or the actual methanol fractions and volumetric flows in SFC was investigated by measuring the mass flow respective pressure and by calculations of the actual volume fraction of methanol. The result revealed a significant difference between the value set and the actually delivered volumetric methanol flow rate, especially at low modifier fractions. If relying only on the set methanol fraction in the calculations, the methanol layer thickness should in this system be highly overestimated.Finally, retention times for a set of solutes were measured and related to the findings summarized above concerning methanol adsorption. A strongly non-linear relationship between the logarithms of the retention factors and the modifier fraction in the mobile phase was revealed, prior to the established monolayer. At modifier fractions above that required for establishment of the methanol monolayer, this relationship turns linear which explains why the solute retention factors are less sensitive to changes in modifier content in this region.

Revisiting the reaction kinetics of selective hydrogenation of phenylacetylene over an egg-shell catalyst in excess styrene

Selective hydrogenation of phenylacetylene is an important reaction for the removal of a small amount of phenylacetylene from styrene monomer. In this work, the kinetics of phenylacetylene hydrogenation over an egg-shell Pd/Al2O3 catalyst in the presence of excess styrene was investigated by using a laboratory-scale fixed-bed reactor. The liquid-phase residence time distribution (RTD) in the reactor under different operating conditions was measured by tracer pulse method, and the Peclet number was determined based on the experimental data and the RTD model. The kinetic experiments showed that an increase in temperature and pressure or a decrease in the liquid hourly space velocity gave rise to increased conversion of phenylacetylene and decreased concentration of styrene. A rigorous mathematical model combining reaction kinetics, external mass transfer, intraparticle diffusion as well as axial dispersion of the liquid phase was developed to describe the selective hydrogenation of phenylacetylene in the reactor, and the kinetic and adsorption parameters involved in the kinetic model were estimated by minimization of the sum of squares of relative residuals between observed and model-derived concentrations of different components. The kinetic model can describe the phenylacetylene hydrogenation over the Pd/Al2O3 catalyst very well, and the activation energies for the hydrogenation reactions of phenylacetylene to styrene, styrene to ethylbenzene and phenylacetylene to ethylbenzene were 48.6, 52.2 and 57.9kJ/mol, respectively.

Determination of void volume in normal phase liquid chromatography

Void volume is an important fundamental parameter in chromatography. Little prior discussion has focused on the determination of void volume in normal phase liquid chromatography (NPLC). Various methods to estimate the total void volume are compared: pycnometry; minor disturbance method based on injection of weak solvent; tracer pulse method; hold-up volume based on unretained compounds; and accessible volume based on Martin's rule and its descendants. These are applied to NPLC on silica, RingSep and DNAP columns. Pycnometry provides a theoretically maximum value for the total void volume and should be performed at least once for each new column. However, pycnometry does not reflect the volume of adsorbed strong solvent on the stationary phase, and so only yields an accurate void volume for weaker mobile phase conditions. 1,3,5-Tri-t-butyl benzene (TTBB) results in hold-up volumes that are convenient measures of the void volume for all eluent conditions on charge-transfer columns (RingSep and DNAP), but is weakly retained under weak eluent conditions on silica. Injection of the weak mobile phase component (hexane) may be used to determine void volume, but care must be exercised to select the appropriate disturbance feature. Accessible volumes, that are determined using a homologous series, are always biased low, and are not recommended as a measure of the void volume.

Sorption Isotherms of Ternary Eluents in Reversed-Phase Liquid Chromatography

The experimental technique of mass spectrometric tracer pulse chromatography was used to measure the excess volume of each eluent component for binary and ternary mixtures of water, acetonitrile, and methanol on a C(18)-bonded silica RPLC packing over the full composition range. The tracer pulse method allowed the direct measurement of excess volumes of each eluent component without numerical integration, assumed isotherm equation, detector calibration, or off-line analysis of the eluent composition. Absolute isotherms were estimated from the experimental data for excess volumes by use of various strategies for the estimation of the volumes of the stationary and mobile phases in dynamic equilibrium with eluents of varying composition. The results indicate that all three eluent components interacted with the alkane bonded phase. Some components were selectively taken up as part of the stationary phase while other components were selectively excluded so the composition of eluent in or on the stationary phase often significantly differed from the composition of the bulk eluent. The exact composition of the stationary phase (bonded phase plus immobilized eluent) was dependent upon the type and composition of the bulk eluent.

Development of the Tracer-Pulse Method for Adsorption Studies of Analyte Mixtures in Liquid Chromatography Utilizing Mass Spectrometric Detection

The tracer-pulse method provides the real adsorption data points directly from simple, straightforward calculations and is therefore a superior method for multicomponent adsorption isotherm determination in HPLC. Only one important problem has restricted its use so far: the tracer peaks are invisible using any conventional detection principle. We present a solution to this problem with an approach with a firm base in analytical chemistry, utilizing stable isotopes and mass spectrometric detection. The new approach was used for the determination of binary adsorption isotherms, and a systematic investigation was made of its main sources of error. With this modification, the tracer method can be a prime choice for future characterizations of multicomponent separation systems and of competitive drug binding studies.

Validation of the Tracer-Pulse Method for Multicomponent Liquid Chromatography, A Classical Paradox Revisited

The tracer-pulse method was extended and validated for the determination of multicomponent adsorption isotherms in liquid chromatography. Competitive adsorption isotherms can be determined for any number of solutes, up to the column resolution limit. The basic principle is to equilibrate the column with an eluent containing a mixture of the solutes and then measure the migration velocity of each of them through the column. It is easy to calculate the stationary phase concentrations from these velocities, given the eluent composition. As in frontal analysis, real competitive isotherm data are measured using this method, unlike other methods, which only produce parametric estimates. The method was used to measure the binary isotherms of beta-blockers on a Kromasil C8 column. The data were fitted to competitive bi-Langmuir adsorption isotherm functions and was found to agree well with the results of frontal analysis and the perturbation method. Computer simulations based on the isotherm parameters were performed and displayed very good agreement with the experimental chromatograms. An intriguing and seemingly paradoxical property is visualized and discussed: the fact that the injected molecules are not found in the detected peaks.

A tracer pulse method for the assessment of airflow patterns in a particleboard mill.

A tracer pulse method was used to investigate the spread of airborne formaldehyde from its source into the surrounding air and the distribution of the air supply in a particleboard mill. The contaminant flow was labeled with sulfur hexafluoride, and the supply of outdoor air with nitrous oxide. The pulse responses of the injected tracer gases were interpreted in terms of the age concept. The result of improvements in the ventilation system was studied through measurements of the formaldehyde concentration and with tracer gas tests. It was found that the approach used in this paper is applicable to charting airflow patterns in a complex industrial environment.

AN IMPROVED TRACER-PULSE METHOD FOR MEASUREMENT OF GAS ADSORPTION EQUILIBRIA

Gas adsorption equilibria were measured by an improved tracer-pulse method. The experimental technique involved injection of small pulses of radioactive gases into a gas stream in equilibrium with an adsorption column. An experimental technique has been developed for quick and accurate determination of the retention volumes of the injected pulses. Analysis of the retention volumes of these pulses allowed direct determination of the gas adsorption equilibria. The improved method resolved the accuracy problem encountered by other researchers and provided data in excellent agreement with data obtained by a conventional static method

Radioactive tracer pulse method of evaluating fracturing of underground oil shale formations : 17F. US BUR. MINES, RI 7791, 1973, 33P

Radioactive tracer pulse method of evaluating fracturing of underground oil shale formations : 17F. US BUR. MINES, RI 7791, 1973, 33P

The use of residence time distribution data for estimation of parameters in the axial dispersion model

In this paper are presented four convenient methods for determination of the mean residence time and the axial dispersion coefficient of a flow system by analysis of data obtained by means of the imperfect tracer pulse method. The analysis is based upon numerical evaluation of the transfer function and its derivatives for a number of values of the Laplace transform parameter, Sτ.

On the use of the imperfect tracer pulse method for determination of hold‐up and axial mixing

AbstractThe imperfect pulse method is a powerful technique for experimental analysis of chemical engineering flow systems. The authors have employed this method in studies of holdup and mixing in gas‐liquid fluidized beds. In the present paper are discussed two problems connected with reduction of the experimental data obtained in such studies.Tailing is a phenomenon that may seriously affect the accuracy of mixing coefficients determined by the imperfect pulse method. Accurate methods of data reduction, based upon analysis of moments, have been developed, and their applicability is demonstrated by analysis of experimental data.Interphase transfer of tracer material is of particular importance in investigations of gas phase mixing in a gas‐liquid system where absorption of tracer gas into the liquid phase normally takes place. A semiempirical method is described for correction of the experimental data in such cases.

Kinetics of the selective hydrogenation of crotonaldehyde to crotyl alcohol

Selective hydrogenation of phenylacetylene is an important reaction for the removal of a small amount of phenylacetylene from styrene monomer. In this work, the kinetics of phenylacetylene hydrogenation over an egg-shell Pd/Al2O3 catalyst in the presence of excess styrene was investigated by using a laboratory-scale fixed-bed reactor. The liquid-phase residence time distribution (RTD) in the reactor under different operating conditions was measured by tracer pulse method, and the Peclet number was determined based on the experimental data and the RTD model. The kinetic experiments showed that an increase in temperature and pressure or a decrease in the liquid hourly space velocity gave rise to increased conversion of phenylacetylene and decreased concentration of styrene. A rigorous mathematical model combining reaction kinetics, external mass transfer, intraparticle diffusion as well as axial dispersion of the liquid phase was developed to describe the selective hydrogenation of phenylacetylene in the reactor, and the kinetic and adsorption parameters involved in the kinetic model were estimated by minimization of the sum of squares of relative residuals between observed and model-derived concentrations of different components. The kinetic model can describe the phenylacetylene hydrogenation over the Pd/Al2O3 catalyst very well, and the activation energies for the hydrogenation reactions of phenylacetylene to styrene, styrene to ethylbenzene and phenylacetylene to ethylbenzene were 48.6, 52.2 and 57.9 kJ/mol, respectively.