Experimental Residence Time Distribution Curves Research Articles

Liquid Backmixing and Phase Holdup in a Gas−Liquid Multistage Agitated Contactor

Axial backmixing of the liquid phase in a multistage agitated contactor (MAC) is investigated through residence time distribution (RTD) experiments over a large range of variables such as gas velocity, liquid velocity, stirring speed, and the number of stages using an air−water system. The experimental RTD curves are satisfactorily compared with the cascade of stirred tanks with backflow (CTB) model. The backflow ratio of the liquid phase is evaluated. The effects of the operating variables on the backflow ratio are examined. The phase holdup in a MAC is also investigated. Based on the data from the present study and the literature, a correlation is developed to predict the backflow ratio in a MAC for an air−water system operated in co-current manner. It is also observed that low backmixing is achieved in the MAC.

Characterization of liquid phase mixing in turbulent bed contactor through RTD studies

Liquid phase mixing in a turbulent bed contactor (TBC) is experimentally characterized through residence time distribution (RTD) studies. A novel technique is employed to obtain the true exit tracer concentration and to increase the average residence time of the liquid. In this technique, an ideal mixed flow tank (MFT) is included in series with TBC. The RTD of the ideal MFT is deconvoluted from the RTD of the entire system to obtain the RTD of TBC alone. Using this method, the effect of gas and liquid velocities, static bed height, particle size and density, and number of stages on mixing of the liquid phase is studied. The experimental RTD curves are satisfactorily compared with axial dispersion model. Correlations are developed for predicting the Peclet number and the axial dispersion coefficient. It is observed in the present study that conditions close to plug flow of liquid phase can be achieved in multistage TBC.

Use of99mTcO4-and Rhodamine WTas tracers and the mathematical convolution procedure to establish the alarm model in the Almendares River

Both 9mTcO4- and Rhodamine WT were employed as tracers to record the residence time distribution (RTD) in 5 segments located in the Almendares River in Havana City. Recovery calculations showed the conservative behavior of the 99mTcO4- under the field conditions studied. The experimental residence time distribution curves from the second segment were convoluted for the d-Dirac injection at the beginning of the section. The spatial behavior of the conservative prompt spills throughout this river section was established for two extreme flow conditions.

Liquid phase mixing in turbulent bed contactor

Axial mixing of the liquid phase in turbulent bed contactor (TBC) is studied through residence time distribution (RTD) experiments over a large range of variables such as flow rate of gas and liquid phases, static bed heights, diameter and density of particles and number of stages in presence of downcomer using air water system. Since all the liquid exits only through the downcomer, it enables the correct estimation of exit concentration of the tracer. The experimental RTD curves are satisfactorily compared with the axial dispersion model. The Peclet numbers evaluated by axial dispersion model and the Peclet numbers reported in the literature are used to propose a unified correlation in terms of operating and geometric parameters. Correlation is also developed for predicting the axial dispersion coefficient. It was observed in the present study that almost plug flow conditions can be achieved in multistage TBC.

A new approach to the analysis of vessel residence time distribution curves

Mathematical models for the evaluation of residence time distribution (RTD) curves on a large variety of vessels are presented. These models have been constructed by combination of different tanks or volumes. In order to obtain a good representation of RTD curves, a new volume (called convection diffusion volume) is introduced. The convection-diffusion volume allows the approximation of different experimental or numerical RTD curves with very simple models. An algorithm has been developed to calculate the parameters of the models for any given set of RTD curve experimental points. Validation of the models is carried out by comparison with experimental RTD curves taken from the literature and with a numerical RTD curve obtained by three-dimensional simulation of the flow inside a tundish.

Computer aided synthesis of RTD models to simulate the air flow distribution in ventilated rooms

In order to achieve a satisfactory level of hygiene and comfort in ventilated premises and to assess the pollutant transfers, it is necessary to control the air flow distribution. An intermediate approach between predictive numerical simulations and experimental determinations of aerodynamic parameters characterizing air distribution in rooms, is constituted by the systemic approach. This article presents the main principles of this approach, which is based on the residence time distribution (RTD) theory, commonly used in chemical engineering, and gives an illustration of its potential extension to ventilation problems. The aim of the IDTS code developed is to build a model from a combination of elementary systems representing basic ideal flow patterns (perfect mixed flow, plug flow,…). The adjustment of the model lies in the comparison of the response to a stimulus injected into the model with an experimental tracer emission performed in a ventilated room inlet. The general solving strategy adopted, consisting in a two-level dissociated treatment of parametric identification and structural identification of models, is first presented. Then, two problems tackled during the qualification step of the IDTS code are presented to illustrate the applications of this computer-aided design tool for the description and the quantification of air flow patterns and the associated pollutant transfers observed in ventilated indoor spaces, and particularly in nuclear ventilation networks. The comparison between experimental residence time distribution curves recorded in two ventilated laboratory enclosures and the simulated ones shows good agreement. For the last illustrative problem, a comparison with results obtained from a computational fluid dynamics tool has been performed.

Tracer studies and breakage testing in pilot-scale stirred mills

A tracer technique is used to provide parameters that describe mixing and breakage in stirred mills. The results are also used to test the accuracy of mixing and breakage models. Tracer studies have been undertaken using a 39-litre vertical Sala agitated mill and a 4-litre horizontal Netzsch mill. The experimental residence time distribution (RTD) of the mills is analysed both in terms of a single mean residence time and a non-linear least squares fit to an optimal number of perf ect mixers of unequal size in series. Results show a strong dependence of RTD on flow rate, minimal dependence on stirrer speed, and support the concept that the RTD's of liquid tracers and solid tracers subject to breakage are similar. A very accurate match to the experimental RTD curves car be achieved with the multiple uneven mixers in series model. Size distribution results from solid tracer tests are used to determine the breakage characteristics of the pilot-scale Sala mill. The population balance model for a single perfect mixer with steady state hold-up is used as the basis for solution of a constrained non-linear optimisation inverse problem for mill breakage rate and breakage function. Experimental results indicate that the breakage rate is first order as hypothesised. The population balance model using optimal breakage parameters provides a reasonable fit to experimental data for cumulative passing percentage as a function of particle size for discrete times. Both the breakage rates and cumulative breakage functions are roughly power law dependent on particle size. Analysis of the size distribution of breakage products indicates that the mode of particle breakage, as indicated by the tracer breakage parameters, is a function of time. This demonstrates that the assumption of time independent breakage parameters in the population balance model may not be valid. Accurate determination of breakage parameters is strongly influenced by the transport characteristics of the slurry through the mill. The work shows that particle breakage in pilot-scale stirred mills is a complex function of both particle and specific mill characteristics. Therefore, in order to gain insight into the appropriate physical processes at work in an industrial scale mill, it is important to perform experiments with and analyse a system that matches the real mill as closely as possible.

Residence time distribution in a corotating twin-screw extruder

A modeling study for the polymer flow and mixing in a corotating twin-screw extruder is presented. The residence time distribution (RTD) in a fully intermeshing corotating twin-screw extruder was measured, using iron powder as tracer, and an LDPE as flow material. Pulse-type input signal experiments were performed in different working conditions. To fit the experimental data obtained on two different screw profiles, the one- and two-parameter flow models described in literature were tested. The best fitting of the experimental RTD curves was obtained by using a delayed tanks in series model with internal recirculation, or a delayed axial dispersion model. This is consistent with the physical characteristics of the flow in a fully intermeshing corotating twin-screw extruder.

Residence time distribution in a wiped liquid film

The present work concerns the evaluation of experimental residence time distribution (RTD) curves obtained in a wiped liquid film formed on the surface of the evaporator cylinder of a molecular evaporator. The dependent variables in the study are the peripheral load and liquid viscosity. The wiping process is modelled by a cascade of apparatuses with laminar falling film ideally mixed at the exit from each stage in a mixer having zero thickness. The comparison of experimental and model data shows that the model developed explains well the actual processes taking place in the film wiped by a segmented wiper. By comparison of the shape of the experimental RTD curves with theoretical model curves, a criterion can be derived, indicating the efficiency of wiping.

Residence time distribution in a holding tube

The residence time distribution (RTD) of two Newtonian (water and 12% sucrose solution) and two non-Newtonian liquids (0·2 and 0·4% guar gum solutions), at a minimum of five different flow rates each, was determined in a holding tube consisting of 10 sections with a total length of 33 m, at 25±0·5°C. The response to a pulse of NaCl solution was detected by a conductivity transmitter and recorded. The relative variances of the experimental RTD curves were used to evaluate the dispersion and the tanks-in-series models. In general, vessel dispersion numbers were higher in laminar flow. For water in laminar flow; the number of tanks-in-series ranged between 14 and 20, whereas in turbulent flow the number was between 61 and 103. For the sugar solution the number of tanks-in-series was higher. For the 0·2% gum solution data the number of tanks was estimated to be between 18 and 34, while for the 0·4% gum solution data it was estimated to be between 7 and 12. Neither model can be said to be better than the other.

Longitudinal dispersion of droplet phase in single- and multi-stage bubble columns for gas-liquid-liquid systems.

The longitudinal dispersion coefficient of droplet phase was measured in 6.6 and 12.2 cm i.d. single- and multistage bubble columns where kerosene was dispersed by air and water flows. Experimental residence time distribution curves of the droplet phase were analyzed by a newly proposed dispersion model, which included the back-flow of droplets through perforated baffle plates and the exchange of dye concentration in the droplet phase at the perforated baffle plates due to dispersion-coalescence phenomena. The exchange coefficient of the dye concentration was obtained by comparing experimental residence time distribution curves with the theoretical ones. The overall longitudinal dispersion coefficient of the droplet phase in multi-stage bubble columns was satisfactorily simulated by the present model, parameters of which were all given as experimental equations. The mean gas holdup in the 12.2cm i.d. column for air-water-solvent (n-hexane, terpentine oil and a mixed oil) systems was increased about 20% by installation of perforated baffl plates.