Gas Residence Time Distribution Research Articles

3-Dimensional Simulation of Air Mixing in the MSW Incinerators

Abstract Combustion control strategies to minimize the pollutants emission from municipal solid waste (MSW) incinerators are formulated based on the improved mixing of air with the products of incomplete combustion and the subsequent increase in oxidative destruction. Secondary air injection into the combustion chamber plays the key role in this mixing process. However, design variables of the air jet into the combustion gas stream are not clearly identified, and the performance of mixing and reaction is not fully understood. Three-dimensional flow simulation was performed to study the mixing performance of a full-scale incinerator combustion chamber according to the secondary air nozzle configuration. A detailed flow was analyzed and the degree of mixing was quantitatively evaluated by introducing a statistical parameter based on the chemical species distribution. The gas residence time distribution was analyzed using the particle trajectory. The overall flow field was strongly influenced by the nozzle configuration. It was demonstrated that the degree of mixing could be improved by selecting larger inter-jet spacing and stronger jet velocity. The staggered arrangement of two opposite nozzle arrays was found to be more effective in terms of mixing and gas residence time distribution.

The effect of bubbling regime on gas and liquid phase mixing in bubble column reactors

An experimental study has been aimed at linking the extent of axial mixing in gas and liquid phases in tall bubble column reactors with the respective bubbling regimes (homogeneous, transition, heterogeneous) encountered in such reactors and with corresponding macro-scale flow patterns of both phases. The experimental programme, carried out in a bubble column reactor of 0.14 m in diameter and 4.1 m in height, included determination of gas and liquid phase residence time distribution (RTD) characteristics for different modes of primary gas dispersion and flow visualisation experiments aimed at identification of macro-scale flow patterns in the reactor. The experimental results proved an essential effect of gas dispersion mode (bubbling regime) on the extent of gas and liquid phase mixing in the reactor. The respective dependences of Pe G and Pe L on the superficial gas velocity, obtained with the distributing plate generating sequentially homogeneous, transition and heterogeneous bubbling regimes, indicated very sensitively the transition from the homogeneous to the heterogeneous bubbling mode (the onset of the transition bubbling regime) occurring at u 0G = 0.04 m s −1 with the appearance of local macroeddies in the bed. The comparison of our mixing data with corresponding dependences of gas holdup and k La L on the superficial gas velocity revealed the existence of direct links between the variations of RTD and mass transfer characteristics induced by the changes of gas dispersion (bubbling) mode. The results of the analysis of variances of gas phase RTD curves justified (in agreement with corresponding Pe G data) the plug flow approximation for the gas phase in the homogeneous bubbling regime and indicated the convective mechanism of gas mixing within the whole region of heterogeneous bubbling conditions. Analogically, the data for the liquid phase yielded the convective mechanism of liquid mixing within the whole range of experimental conditions. The mixing data obtained for both phases in the heterogeneous bubbling regime were adequately described by the model of consecutive circulation cells with the backflow between adjacent cells, proposed on the basis of the flow visualisation experiments as a realistic simplified representation of the macro-scale flow structures in heterogeneous bubble beds.

Interpretation of gas residence time distributions in large airlift tower loop reactors

Gas-residence time distribution (RTD) response curves measured in a 23 m high pilot plant airlift tower loop reactor, which consisted of a riser, a special downcomer construction and at the top of the riser a large head. The measurements were evaluated by means of a deterministic dispersion model, which yielded the following particular parameters for the riser, downcomer and the head: Gas-Bo numbers, gas-mean residence times, gas holdups, liquid velocities, gas and liquid circulation times as well as a fraction of the large and small bubbles in a model medium (water) and during cultivation of baker's yeast.

Interpretation of the gas residence time distributions in large stirred tank reactors

The behaviour of dispersed gas in large aerated stirred tank reactors is modelled by means of a Markov-process, which distinguishes between small recirculation bubbles with “stagnant gas” content, large rising bubbles with “active gas” content and exchange of stagnant and active gas contents, the “gas exchange” region at the impeller. The measurements of the gas residence time distributions (RTDs) in an 1.5 m3 aerated stirred tank reactor with water and Penicillium chrysogenum cultivation medium are interpreted by this model.

Interpretation of the gas residence time distributions in large stirred tank reactors

Interpretation of the gas residence time distributions in large stirred tank reactors

Interpretation of gas residence time distributions in large airlift tower loop reactors

Interpretation of gas residence time distributions in large airlift tower loop reactors

Neue Erkenntnisse zum Mischverhalten von Submersreaktoren

AbstractMixing behaviour of submersed reactors. New improved measuring techniques lead to new insights into the local mixing behaviour of submerse reactors. As exemplified by airlift tower loop reactors, it is shown that it is possible to obtain more information from gas residence time distributions if the signal‐to‐noise ratio of the measurement can be improved significantly. Additional information is obtained, e.g., on that part of the gas recirculated around the loop or the circulation time of the gas phase. The recirculated gas significantly influences the gas backmixing in such reactors. A more thorough investigation of the gas phase motion requires locally resolved data. For the properties of the gas phase, local measuring techniques are available, which allow for measurements of the mean bubble velocity profile even during production runs in large aerated stirred tank reactors. The results show that the gas backmixing is acutely dependent on the viscosity of the continuous liquid phase. Complementary measurements on the properties of the continuous liquid phase can be investigated by means of the heat‐pulse‐technique. This also is applicable in real media, e.g., during biotechnological cultivation runs. Besides the measurement of the mean liquid velocity, one can, furthermore, obtain some information on the locally dominating mixing mechanism. One significant result of such investigations is that in the airlift reactors investigated the bubble‐wake mixing mechanism dominates, whereas the isotropic turbulence in terms of the statistical turbulence theory plays a secondary role. This even holds at very high energy inputs.

Gas residence-time characteristics in a high-velocity circulating fluidised bed of FCC catalyst

Gas residence-time distributions have been measured in a 0.09-m ID high-velocity bed of FCC catalyst. A dynamic gas tracer system based on a pair of rapid-response mass spectrometers was employed, and the tracer itself was argon delivered in the form of a pulse. Concentration—time responses were recorded at two elevations, and system transfer characteristics were extracted in the form of variance increase and peak-to-peak velocity. The variance change in the bed was found to decrease with increasing gas velocity and to increase initially with increasing solids circulation. This increase with a rise in solids flux appeared to weaken at high solids rates. Peak-to-peak velocities were found to display a maximum at a superficial gas velocity of around 4 – 5 m/s and, when the variance data are processed into the form of axial dispersion coefficients, a similar peak emerges. One possible explanation is that a radially symmetrical core/annulus structure exists at low velocities, and that beyond about 5 m/s this structure gives way to a more random lateral distribution of lean and dense regions. A possible distinction between “fast fluidisation” and “riser transport” is suggested on this basis.

Residence time distribution of gas in the fluidized bed in a rotating drum.

When a tumbling particle bed in a horizontal rotating drum is aerated, the particle bed can be considered as a gas fluidized bed, which may be called a fluidized bed in rotating drum (FBRD).Measurement of the residence time distribution (RTD) of gas in an FBRD of coarse particles was carried out by the stimulus-response method using a bench-scale rig. The concentration change of nonadsorbing tracer gas (methane) in the exit stream, as the response to a pulse input, was measured by a flame ionization detector connected with PC data acquisition system.The results of the experiments revealed that the RTD of gas was affected by the parameter ξ, which included aeration rate and rotation speed of drum, and aeration angle. Backmixing of gas was found for large ξ. The reactor efficiency, calculated from the real RTD functions, was close to that of the packed bed and was distinctly higher than that of the conventional bubbling fluidized bed for small ξ. Finally, the reactor efficiency of the FBRD was represented in terms of parameter ξ.

Gas flow in a pressurized fluidized-bed combustor

The residence time distribution (RTD) of gas in a pressurized fluidized-bed combustor and in the freeboard have been measured with 41Ar as radioactive tracer. The procedure of normalisation of measured impulse responses could be used to evaluate the RTD in the whole combustor. A deconvolution method using Laguerre functions was successfully used for the analysis of RTD in the freeboard. In the whole combustor and in the freeboard region a tendency to decreasing mean gas residence time with increasing pressure has been found. A simple method for estimation of fast bubble flow is presented. The bubble flow rate and rise velocity were estimated from the various RTDs of gas measured for the same operating conditions. The expected tendency for bubble flow to decrease with pressure has been confirmed. The results agree with values for bubble rise velocity measured independently with an impedance probe.

A comparison of some methods of estimating volumetric mass‐transfer coefficients in an external‐loop airlift fermenter

Volumetric mass-transfer coefficients were measured in an 11-L external-loop airlift fermenter with deionized water, a fermentation medium, and during a fermentation. Both a Mackareth oxygen electrode and a novel rapid-response probe were used. When the conventional step-change dynamic method was used for water, the long, nonlinear response time of the Mackareth electrode made correction of its readings difficult; this problem did not occur when the rapid-response probe was used. A comparison was made with a method of mass-transfer coefficient determination which does not involve any assumptions about the gas residence time distribution. However, this method requires that the liquid phase is well-mixed and this requirement was not met in the airlift fermenter. Comparison of the present results with other K(L) a determinations for airlift fermenters showed that K(L) a in water depends on the active gas holdup, the value of K(L) a/epsilon at 20 degrees C being ca. 0.37 s(-1). Although higher gas holdups were obtained with the fermentation medium than for water, the values of K(L) a/epsilon were lower, ca. 0.22 s(-1) at 20 degrees C.

The measurement of gas residence time distribution in a pressurized fluidized-bed combustor using 41Ar as radiotracer

Applying 41Ar as tracer the residence time distributions of gas in a pressurized fluidized-bed combustor operating at up to 8.4 bar and 980°C have been measured. The tracer amount necessary for a measurement was estimated using a triangle approximation of the mentioned distribution. Good reproducibility was obtained for mean residence time t ̄ and somewhat less good for the variance of the residence time distribution [ σ 2( θ)]. Both t ̄ and 1 σ 2(θ) decreased with increasing pressure and bed content. At 8 bar the behaviour of the combustor approaches that of a perfect mixer.

Gas residence time distribution in stirred tank bioreactors

Residence time distributions of gas phase have been measured in fermentation broths and in simulated media of low as well as high viscosity (1 × 10 −3 − 0.5 Pas) using a step-function-Helium-tracer technique. On the basis of these, simple mathematical models have been developed and their parameters are qualitatively correlated with operating conditions. It was shown that even in low viscosity Newtonian fluids, gas phase does not behave as well mixed. Differences between gas flow patterns in low and high viscosity fluids, already reported in literature, have been verified.

THE ROLE OF GAS DISTRIBUTION IN FLUIDIZED BED CHEMICAL REACTOR DESIGN†

Abstract The design of fluid bed gas distributors may have a marked influence on the performance of a fluid bed reactor. The primary physical reason for this influence is that the distributor design influences the hydrodynamics and thus the gas/solid contacting pattern in the fluidized bed. In the paper presented here the influence of distributor design on mass transfer and chemical reaction has been investigated systematically in fluid bed reactors with diameters of 0.2 and 1.0 meter. Coefficients of mass transfer between the bubble phase and the suspension phase were determined from chemical conversion and tracer gas residence time distribution measurements. In the experimental program the height of the fluidized bed was varied between 0.3 m and 0.9 m with superficial gas velocities in the range of 0.06 m/s to 0.30 m/s. The comparison of the experimental results with a suitably modified and extended two-phase model yields quantitative relationships which allow to account for the influence of the gas dis...

A dynamic response technique for the estimation of gas—liquid mass transfer coefficients in a stirred vessel

A method is described for evaluating mass transfer coefficients, k La , in gas sparged stirred vessels. It involves measurement of both liquid and gas concentration responses following a step change in inlet gas concentration, and differs from previously published methods in that the k La evaluation is independent of the gas residence time distribution. Results are presented that demonstrate the suitability and consistency of the method for a range of gas mixing levels. A modification, that involves only a knowledge of the early liquid concentration response, is suggested as worthy of further consideration.

Gas residence time distributions in single and multistage fluidized beds with porous particles II. Longitudinal dispersion

The influence of the following parameters on the longitudinal dispersion Peclet number was investigated: flow rate, temperature, gas distributor type, number of stages, the type of the compartment separating plates and the height to diameter ( H/D) ratios of the bed. A bench-scale fluidized bed with porous particles was used as a reactor with two different types of tracers, argon (slightly adsorbing component) and carbon dioxide (strongly adsorbing component). All of these parameters, with the exception of the temperatures and H/D ratio influence Pe when argon is used. When CO 2 is used, no influence of the temperature and H/D ratio on Pe was found to be significant. However, Pe was found to be slightly dependent on the gas distributor type but strongly dependent on the number of stages employed. This behaviour of Pe with regard to Ar and CO 2 is explained by means of a simple model of the fluidized bed.

Gas residence time distributions in single and multistage fluidized beds with porous paricles I. Mean Residence Time

The influence of flow rate, temperature, gas distributor type, number of stages, type and spacing of the compartment separating plates and bed height to diameter ( H/D) ratio on the mean gas residence time was investigated for a slightly adsorbing (Ar) and strongly adsorbing (Co 2) gas in bench-scale fluidized beds with porous particles. With Ar, only the flow rate and H/D ratio influence τ. With Co 2 the temperature, the gas distributor type, the number of stages and the type of the compartment separating plates also influence τ. With increasing temperature the effect of the number of stages on τ becomes less and/or disappears. The behaviour is explained by means of a simple model of the fluidized bed.

Residence time distribution of gas in spouted beds

AbstractStimulus‐response experiments have been carried out, using helium gas as tracer, to measure the residence time distribution of gas in spouted beds. The data obtained, under a variety of experimental conditions, are compared against predictions from a two‐region gas flow model. The model postulates plug flow of gas in the spout and axially‐dispersed plug‐flow in the annulus, the extent of axial dispersion being accounted for by a coefficient, D, which is an adjustable parameter of the model. The values of D thus determined were found to increase with gas flow rate, to be relatively insensitive to bed path, and to be much larger for beds of wheat than of polystyrene.

A stochastic model for residence time and contact time distributions of the gas in multistage fluidised beds

A stochastic model based on Markov chain is proposed to describe the gas residence time distribution and contact time distribution in multistage fluidised beds. The Kolmogorov—Feller equations are solved by Laplace transform and subsequent numerical inversion. The effects of flow fluctuations on the distributions are discussed and the theoretical model is compared with the experimental data in literature.

Gas residence time distribution in multistage fluidised beds

The residence time distribution of the gas in multistage fluidised beds is studied, using Helium as tracer, for different conditions of the solids concentration, the number of stages and the height-to-diameter ratio of the stage. The results are compared with that of batch fluidisation with and without baffles and it is found that continuous fluidisation gives better flow characteristics of the gas and possesses a wider range of operation than batch fluidisation.