Residence fluid dynamic

Computer Models, The actual residence time for waste destmction can be quite different from the superficial value calculated by dividing the chamber volume by the volumetric flow rate. The large activation energies for chemical reaction, and the sensitivity of reaction rates to oxidant concentration, mean that the presence of cold spots or oxidant deficient zones render such subvolumes ineffective. Poor flow patterns, ie, dead zones and bypassing, can also contribute to loss of effective volume. The tools of computational fluid dynamics (qv) are useful in assessing the extent to which the actual profiles of velocity, temperature, and oxidant concentration deviate from the ideal (40). 

Third, design constraints are imposed by the requirement for controlled cooling rates for NO reduction. The 1.5—2 s residence time required increases furnace volume and surface area. The physical processes involved in NO control, including the kinetics of NO chemistry, radiative heat transfer and gas cooling rates, fluid dynamics and boundary layer effects in the boiler, and final combustion of fuel-rich MHD generator exhaust gases, must be considered. 

Measurements of the true reaction times are sometimes difficult to determine due to the two-phase nature of the fluid reactants in contact with the solid phase. Adsorption of reactants on the catalyst surface can result in catalyst-reactant contact times that are different from the fluid dynamic residence times. Additionally, different velocities between the vapor, liquid, and solid phases must be considered when measuring reaction times. Various laboratory reactors and their limitations for industrial use are reviewed below. 

This study investigates the hydrodynamic behaviour of an aimular bubble column reactor with continuous liquid and gas flow using an Eulerian-Eulerian computational fluid dynamics approach. The residence time distribution is completed using a numerical scalar technique which compares favourably to the corresponding experimental data. It is shown that liquid mixing performance and residence time are strong functions of flowrate and direction. 

Other fluid dynamic models of slurry flow have also been developed by other workers [57]. Coppeta, Rodgers, Radzak, and coworkers examined slurry flow, both from a simulation point of view, and from an experimental angle [6,10,11]. A special test apparatus is used consisting of flourescent injections of die that is entrained beneath a glass wafer enabling observation of slurry flow patterns and residence time. Such studies are instrumental... 

In practical combustion systems, such as CO boilers, the flue gas experiences spatial and temporal variations. Constituent concentration, streamline residence time, and temperature are critical to determining an efficient process design. Computational fluid dynamics (CFD) modeling and chemical kinetic modeling are used to achieve accurate design assessments and NO, reduction predictions based on these parameters. The critical parameters affecting SNCR and eSNCR design are listed in Table 17.4. 

In macroscopic reactors, knowledge of the velocity profile in the channel cross-section is a necessary and sufficient prerequisite to describe the material transport. In microscopic dimensions down to a few micrometers, diffusion also has to be considered. In fact, without the influence of diffusion, extremely broad residence time distributions would be found because of the laminar flow conditions. Superposition of convection and diffusion is called dispersion. Taylor [91] was among the first to notice this strong dominating effect in laminar flow. It is possible to transfer his deduction to rectangular channels. A complete fluid dynamic description has been given of the flow, including effects such as the influence of the wall, the aspect ratio and a chemical wall reaction on the concentration field in the cross-section [37]. 

The key reactive separation topics to be addressed in the near future are a proper hydrodynamic modeling for catalytic internals, including residence time distribution account and scale-up methodology. Further studies on the hydrodynamics of catalytic internals are essential for a better understanding of RSP behavior and the availability of optimally designed catalytic column internals for them. In this regard, the methods of computational fluid dynamics appear very helpful. 

The dry-based capacity of the equipment is 1.78 kg s-1, while the recovery efficiency of the particles is 98-99%. It is reported that the conditions of fluid dynamics are very stable and that the equipment can be operated with a very high concentration of dense phase. It is clear that at least one of the researchers major intentions in the design of the dryer is to lengthen the residence time of the particles. 

One reason to use micro structured reaction chambers is certainly the possibility of describing the fluid dynamic behavior in these structures due to the laminar flow regime. With the following calculations the reactive gas flow in a square micro structure with coated catalytically active walls will be studied in detail. The task was to find a channel arrangement and to calculate the residence time distribution of this arrangement numerically (Figure 4.93). 

Campbell s series [5] states that many authors recommend, as do I, the O Connell method for hydrocarbons. You may select the two-film method by entering T. The Fractionation Research Institute (FRI) has for more than 30 years used and proven a much more thorough method called the two-film method, which is equally applicable to both fractionators and absorbers [6], The FRI method accounts for the actual tray internal configuration and fluid dynamics, making it far superior to the O Connell method. I have therefore produced a type of two-film method for determining tray efficiency that has been checked to be within 3% accuracy of hundreds of answers calculated with the FRI method. Called the Erwin two-film method, it is recommended for all types of trays and will also calculate the liquid tray residence time in seconds. This method is included in the three tray computer program executable files in the CD accompanying this book. 

In a continuous reaction process, the actual residence time of the reaction partners in the reactor plays a major role. It is governed by the residence time distribution of the reactor which gives information on back-mixing (macro-mixing) of the throughput. This emphasizes the interaction between chemical reaction and fluid dynamics. 

Roughly, an increase of residence time x by a factor of ten, results in a decrease of fuel content of 1 % 2% provided that all fluid dynamic conditions are kept constant. 

It was shown that the efficiency for radial mixing depends on the gas phase as well (O. Eichstadt, Continuous Mixing of Fine Particles within Fluid Dynamic Vertical Tube Mixers, Dissertation, in German, ETH-Zurich, 1997). At best they operate with low volume concentration and for particles between 20 and 200 pm. Static mixers have been used for very abrasive free-flow materials such as silicon carbide. Since any rotating equipment is avoided inside static mixers, abrasion is limited. As will be shown below, mixture quality is dependent on feed consistency and residence time within the static mixer. Since the latter is very short in static mixers (seconds or fractions of a second), short-time feeding precision has to be very high to achieve high-quality mix. 

Following the concept of Fig. 6.9, the parameters describing the fluid dynamic and residence time distribution of the plant peripherals are determined by means of pulse experiments. For this purpose, a small amount of tracer is injected into the plant without the column and the output concentration is measured. 

Use of oxygen in the reactor calls for adequate consideration of airflow (air plus oxygen) volume, as this can imbalance the fluid dynamics, causing a certain distribution of particles sizes to become airborne and adversely affecting the residence time and product quality. 

Table 3 Computational fluid dynamics biomass particle size impacts—residence time, CE, and fly ash/bottom ash partitioning for 10% switchgrass coflring at full load in a four-level burner 150 MWe t-fired boiler...

Although both expressions are commonly used, they fail to predict some important macroscopic properties of solid-fluid suspensions, such as the expansion and sedimentation profiles. To overcome this limitation Mazzei Lettieri (2007) developed a relationship for the drag coefficient that is based on the empirical correlation by Richardson Zaki (1954) describing the expansion profiles of homogeneous fluid-solid suspensions. Its main feature resides in the fact that the expression is consistent with the Richardson and Zaki correlation over the whole range of fluid-dynamic regimes and for any value of the suspension void fraction. It has the following formulation ... 

E/N, f(u) as well as of the fluid dynamics of the chemical discharge reactor i.e., type of flow (plug, laminar, turbulent) and related dimensionless quantities relevant to the definition of (a) velocity profiles and corresponding effective residence times,... 

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