Measurements and modeling of heat generation in a trickling biofilter for biodegradation of a low concentration volatile organic compound (VOC)

The experimental and theoretical heat generation behavior of a trickling biofilter treating toluene is discussed. The experimental results show that the temperature of the packed bed has a significant effect on the purification performance of the trickling biofilter and that an optimal operation temperature exists between 30 and 40 °C. During the gas–liquid co-current flow, the temperature in the packed bed gradually rises along the direction of the gas and liquid flow due to the exothermic biodegradation of toluene. The temperature rise between the inlet and outlet of the trickling biofilter increases with an increase in the gas flow rate and inlet toluene concentration. In addition, a larger liquid flow rate leads to a smaller temperature rise. The heat generation process occurring in the trickling biofilter is modeled by representing the packed bed as an equivalent set of parallel capillary tubes covered by the biofilm. The temperature profile in the packed bed during the liquid–gas co-current flow is analyzed by simultaneously solving the problem of gas–liquid two-phase flow and heat and mass transfer through the liquid film and biofilm. It is shown that the model agrees well with the experimental data, predicting the variations of the temperature rise between the inlet and outlet of trickling biofilter with the increasing gas and liquid flow rates.     

Propagation of a cylindrical shock wave in a mixture of a non-ideal gas and small solid particles under the action of monochromatic radiation

A cylindrical shock wave in a dusty gas under the action of monochromatic radiation into the stellar atmosphere with a constant intensity per unit area is discussed. The gas is assumed to be grey and opaque, and the shock is assumed to be transparent. The dusty gas is considered as a mixture of a non-ideal gas and small solid particles. To obtain some essential features of shock propagation, small solid particles are considered as a pseudo-fluid, and it is assumed that the equilibrium flow condition is maintained in the entire flowfield. The effects of the parameters of the gas non-idealness, the mass concentration of solid particles in the mixture, the ratio of the density of solid particles to the initial density of the gas, and the radiation parameter on flow variables are investigated. It is shown that an increase in the gas non-idealness and the radiation parameter has a decaying effect on the shock waves, whereas the shock strength increases with an increase in the ratio of the density of solid particles to the initial density of the gas. It is found that an increase in the gas non-idealness and the ratio of the density of solid particles to the initial density of the gas has the opposite effects on the fluid velocity, pressure, and shock strength. It is also shown that an increase in the radiation parameter has a trend to decrease the flow variables and the shock strength.     

Velocity distributions in a gaseous plasma: I. Derivation of flow types within a helium plasma

Velocity distributions in a plasma jet are generally determined by means of direct methods which are liable to alter the medium, especially at very high temperatures. A gas flow model is proposed, which is worked out from the luminous profile of a helium plasma jet, such as viewed through a dense orange glass filter. The knowledge of the luminous profile equation, temperature distribution, gas flow rate and energy balance completely determine the velocity distribution in the plasma. Boundary conditions lead, in certain cases, to the concept of an axial turbulent core. The helium plasma was operated at 27 kW, with a gas flow rate of 7.2 g/minute. The jet was issuing into an atmosphere kept at 300 mm Hg in order to favor conditions leading to laminar flow.     

Anisokinetic sampling of aerosols at a slot intake

The flow at intake to some aerosol samplers may be regarded as two-dimensional flow into a thin-walled slot. A theoretical analysis is given of the efficiency with which such a slot collects particles from an approaching gas flow for the case where the approach velocity exceeds the mean velocity in the intake. In the theory, which takes regions of separated flow into account, the gas flow is treated as a potential flow and Stokes law is assumed to govern the interaction between particles and gas.

Curves of calculated collection efficiency are given over a wide range of Stokes number for ratios of approach velocity to mean intake velocity up to 20. Where Stokes law is not applicable the collection efficiency obtained from the curves is an upper limiting value.     

Liquid phase residence time distribution for a two-phase countercurrent flow in a packed column with a novel internal

The liquid phase residence time distribution (RTD) for gas–liquid countercurrent flow in a packed column with a novel internal was measured by conductivity measurements and an air–water system. The RTD of a liquid tracer is well represented by the ADM and PDE models. At lower gas flow rates, the Peclet number of the liquid in the packed column with the internal is lower than that without the internal; at higher gas flow rates, it is vice versa, especially with an internal with a higher volume fraction. The distribution of the liquid RTD can be improved by using suitable geometric parameters of the internal to give a larger volume fraction and a lower stage height.     

Analysis of a model for a nonisothermal continuous fluidized bed catalytic reactor

A three-phase model, consisting of a dilute or bubble phase, an interstitial gas phase and a solids phase, has been developed for a non-isothermal gas fluidized-bed catalytic reactor with continuous circulation of catalyst particles. The dilute phase is assumed to be in plug flow, the interstitial gas is considered to be either perfectly mixed or in plug flow, and the particles are assumed to be perfectly mixed.It is shown that the conversion in a reactor and steady state temperature and concentration profiles are the same irrespectively of the assumed flow pattern in the interstitial gas.The numerical computations were done for the case of a single irreversible reaction with or without catalyst decay and for the case of two consecutive irreversible reactions. In the case of an adiabatic reactor and ordinary exothermic reactions, or in the case of a cooled reactor with a highly exothermic reaction, multiple steady states may occur.     

Study of gas cavity size hysteresis in a packed bed using DEM

When a high velocity gas jet is introduced into a packed bed a cavity is formed. The size of the cavity shows hysteresis on increasing and decreasing gas flow rates. This hysteresis leads to different cavity sizes at same gas flow rate depending on the bed history. The size of cavity affects the gas flow profiles in the packed bed. In this study the cavity size hysteresis phenomenon has been modeled using discrete element method along with turbulent gas flow. A reasonable agreement has been found between computed and experimental results on cavity size hysteresis. The effect of various parameters, such as nozzle height from the bed bottom and packing height, on the cavity size hysteresis has been studied. It is found that inter-particle interaction forces along with gas drag and bed porosity play an important role in describing the cavity size hysteresis. The injection of gas flow allows the particles to go to an unconstrained state than they were previously in, and their ability to remain in that state, even under decreased gas drag force, leads to the phenomenon of cavity size hysteresis.     

Discrete particle simulation of gas–solid flow in a blast furnace

Gas–solid flow plays a dominant role in the multiphase flow in an ironmaking blast furnace (BF), and has been modelled by different approaches. In the continuum-based approach, the prediction of the solid flow pattern remains difficult due to the existence of the stagnant zone in the BF lower central part. This difficulty has recently been shown to be overcome by discrete particle simulation (DPS). In this work, the DPS is extended to couple with computational fluid dynamics (CFD) to investigate the gas–solid flow within a BF. The results demonstrate that the DPS–CFD approach can generate the stagnant zone without global assumptions or arbitrary treatments. It confirms that increasing gas flow rate can increase the size of the stagnant zone, and in particular changes the solid flow pattern in the furnace shaft. More importantly, microscopic information about BF gas–solid flow, such as flow and force structures that are extremely difficult to obtain in continuum-approach or experiments, can be analyzed to develop better understanding of the effect of gas phase, and the underlying gas–solid flow mechanisms.     

Gas-phase residence time distribution in a falling-film microreactor

Industrial-scale performance of gas-liquid reactors can be difficult to optimise for very rapid or highly exothermic reactions. Microstructured reactors for laboratory measurements offer new opportunities for the study of these reactions by enabling precise heat management and fine control of reactor operating conditions. For accurate experimental study, characterisation of the flow conditions within these new reactor devices is essential.The present study examines experimental residence time distributions for the gas phase through a microstructured falling-film reactor, in order to develop an appropriate flow model for further study of gas-phase mass-transfer characteristics in the system. For the gas-phase residence time distribution experiments, the detection system involves a flow of oxygen containing ozone as a tracer gas with continuous monitoring of the concentration by UV-light absorption. The experimental results are used to model the flow behaviour in the gas volume over the gas-liquid contact zone as a series of continuous stirred tank reactors whose number is a simple function of the gas Reynolds number.The experimental results are compared with computational fluid dynamics calculations of the gas flow within the reactor. The comparison indicates a clear correlation of the flow model behaviour with the appearance of recirculation loops in the reaction chamber and the effect of the gas jet at the entrance of the gas-liquid contact zone.     

Vertical pneumatic conveying: a flow regime diagram and a review of choking versus non-choking systems

For vertical pneumatic conveying of granular solids, a flow chart describing two different types of gas—solid systems, viz. the choking system and the non-choking system, is presented. Published equations for the prediction of whether a particular gas—solid tube system is of the choking type or non-choking type are reviewed. For the choking type system, a quantitative flow regime diagram for predicting demarcation between packed bed conveying and slugging dense phase conveying, and demarcation between slugging dense phase conveying and lean phase conveying, is developed in terms of the key parameters of loading ratio, gas and solid velocities. The usefulnes of the flow regime diagram in design is discussed. The shortcomings of an earlier flow regime diagram proposed by Leung [14] are overcome in the present diagram.     

柱形和锥形移动床中气体的扩散特性

采用脉冲氦气体示踪技术研究了柱形和锥形移动床中气相的扩散特性,并与实验结果进行对比. 结果表明,在主体流动区(0.110.89),柱形床会出现向下窜气的现象,而锥形床中向下窜气量降低40%~50%,气相径向流动更趋近平推流. 根据径向气流在柱形和锥形移动床内的流动特点,用实验数据回归了气体浓度分布的无量纲经验关联式,计算值与实验值吻合较好.     

Design and Performance of a Pumped Counterflow Virtual Impactor

Counterflow virtual impaction uses a flow of gas in a direction opposite the motion of the particles to separate them from gas and smaller particles. In the past such devices have used aircraft flight or wind tunnel flow to impart momentum to the particles. Here we describe the design and performance of an apparatus, termed a pumped counterflow virtual impactor (PCVI), which uses a vacuum pump to provide the flow. We show that this device is capable of inertial separation with a sub-micrometer cutoff diameter, particle enhancement approaching the ratio of the output to the input flow, and replacement of the ambient gas in the output flow with another gas that may be more suitable for downstream analysis techniques. Rejection of input gas and particles smaller than the cutoff diameter can exceed 99.9%.     

Transient analysis of gas flow in a straight pipe

In the present work, numerical simulation has been done to analyze transients in gas flow and pressure in a horizontal straight pipe. For a single gas pipeline, eight representative cases corresponding to different causes of transient behaviour are simulated to predict unsteady state flow and the evolution of pressure profiles. The numerical results show that depending upon the pipe dimensions and operating variables such as pressure and gas flow rate, transient effects in the pipeline may last for a long time and/or over significant length of pipe. The simulations predict an initial surge in gas flow rate greater than the final steady‐state value if the pressure drop across the pipe is increased. Similarly, initial flow rate may decrease below the final steady‐state value if the pressure drop is decreased. In case of complete closure of a valve, oscillations in both pressure and mass flow rate are observed, which gradually decay and the steady state conditions of no flow are ultimately achieved. The present results are compared with a published work from the literature. A reasonably good agreement is found between these two predictions. The present study is of practical significance in safe design and operation of a gas delivery system.     

Numerical simulation of horizontal jet penetration in a three-dimensional fluidized bed

The introduction of reactant gas as a jet into a fluidized bed chemical reactor is often encountered in various industrial applications. Understanding the hydrodynamics of the gas and solid flow resulting from the gas jet can have considerable significance in improving the reactor design and process optimization. In this work, a three-dimensional numerical simulation of a single horizontal gas jet into a cylindrical gas-solid fluidized bed of laboratory scale is conducted. A scaled drag model is proposed and implemented into the simulation of a fluidized bed of FCC particles. The gas and particles flow in the fluidized bed is investigated by analyzing the transient simulation results. The jet penetration lengths of different jet velocities have been obtained and compared with published experimental data as well as with predictions of empirical correlations. The predictions by several empirical correlations are discussed. A good agreement between the numerical simulation and experimental results has been achieved.     

Gas flow regime changes in a bubble column filled with a fibre suspension

Gas flow characteristics in opaque fibre suspensions have been captured on film using a stop‐motion X‐ray imaging technique called flash X‐ray radiography (FXR). Gas flows in a bubble column filled with various cellulose fibre suspensions from 0% (an air–water system) to 5% by mass have been observed. The gas flow regime changes from vortical to churn‐turbulent as the fibre concentration increases for a fixed superficial gas velocity. Two new gas flow regimes, identified as surge churn‐turbulent and discrete channel flow, have also been recorded at high fibre concentrations.     

Experimental study and CFD modelling of a two-phase slug flow for an airlift tubular membrane

The aim of the present study was to develop a computational fluid dynamics (CFD) model to study the effect of slug flow on the surface shear stress in a vertical tubular membrane. The model was validated using: (1) surface shear stresses, measured using an electrochemical shear probe and (2) gas slug (Taylor bubble) rising velocities, measured using a high speed camera. The length of the gas slugs and, therefore, the duration of a shear event, was observed to vary substantially due to the coalescing of gas slugs as they travelled up the tube. However, the magnitude of the peak surface shear stress during a shear event was not observed to vary significantly. The experimental conditions significantly affected the extent to which the gas slugs coalesced. More coalescing between gas slugs was typically observed for the experiments performed with higher gas flow rates and lower liquid flow rates. Therefore, the results imply that the frequency of shear events decreases at higher gas flow rates and lower liquid flow rates.Shear stress histograms (SSH) were used as a simple approach to compare the different experimental conditions investigated. All conditions resulted in bi-modal distributions: a positive surface shear peak, caused by the liquid slug, and a negative shear peak caused by the gas slugs. At high gas flow rates and at low liquid flow rates, the frequency of the shear stresses in both the negative and positive peaks were more evenly distributed. For all cases, increasing the liquid flow rate and decreasing the gas flow rate tends to result in a predominant positive peak. These results are of importance since conditions that promote evenly distributed positive and negative peaks, are likely to promote better fouling control in membrane system. At high liquid and low gas flow rates, the frequencies obtained numerically and experimentally were found to be similar, deviating by less than approximately 10%. However, at high gas and low liquid flow rates, the differences were slightly higher, exceeding 20%. Under these conditions, the CFD model simulations over predicted the shear stresses induced by gas slugs. Nonetheless, the results indicate that the CFD model was able to accurately simulate shear stresses induced by gas slugs for conditions of high liquid and low gas flow rates.     

Experimental study of a reverse flow catalytic converter for a dual fuel engine

Theperformance of a reverse flow catalytic converter for a methane/diesel dual fuel engine, a monolith honeycomb converter with palladium catalyst washcoat, was evaluated under steady and transient engine conditions. The reverse flow converter provided superior performance (that is, higher conversion of pollutants) for several steady engine operations, compared with unidirectional flow operation. For transient operation following a step change in engine operating conditions, reverse flow is better than unidirectional flow when the change in engine operation results in a reduction in exhaust gas temperature. For an increasing exhaust gas temperature, reverse flow decreased the rate of increase of reactor temperature. The reverse flow converter was tested using the transient Japanese 6‐Mode tests. Reverse flow operation gave higher conversions than unidirectional flow for this test, with a switch time of 5 s giving the best results.     

Hydrodynamics of a dual fluidized-bed gasifier—Part I: simulation of a riser with gas injection and diffuser

Circulating fluidized beds often apply secondary gas injections and diffusers in the riser. These strongly affect the fluid dynamics of the gas-solid flow in the system. This work is performed to study these effects in a cold flow model of an biomass gasifier. It is shown that in the diffuser there is a bulb of the suspension flow, which enhances the internal solids recirculation by a factor of 3.5. Thus, the solids hold-up and the pressure drop in the diffuser are significantly increased. The study on the effect of gas injection confirms that the solids circulation rate is more enhanced by gas injections in the lower part of the riser than in the upper part. From the investigated operating parameters, the gas flow rates and the particle diameter have the strongest effects on solids circulation and mass distribution in the riser. The effect of riser geometry properties, besides the cross-section areas and the total height, was found to be small.     

An experimental comparison of a graphite electrode and a gas diffusion electrode for the cathodic production of hydrogen peroxide

This work studies the production of hydrogen peroxide through the cathodic reduction of oxygen in acidic medium, by comparing the results obtained using a commercial graphite and a gas diffusion electrode. A low pH was required to allow the application of hydrogen peroxide generation to an electro-Fenton process. The influence of applied potential and the gas flow composition were investigated. The gas diffusion electrode demonstrates a higher selectivity for hydrogen peroxide production, without significantly compromising the iron regeneration, thus making its successful application to a cathodic Fenton-like treatment, possible. Unlike the graphite cathode, the gas diffusion cathode also proved to be effective in the air flow.