Stirred reactors, simulation

Concluding, it is essential to represent complex, real-life flow situations by computationally tractable models that retain adequate details. As an example, a computational snapshot approach that simulates the flow in stirred reactors or other vessels for any arbitrary impeller has been developed [5]. This approach lets the engineer simulate the detailed fluid dynamics around the impeller blades with much less computations that would otherwise be required. Improvements in CFD technique are likely to encourage further work along these lines. 

Bakker, A., Laroche, R.D., Wang, M.H. and Calabrese, R.V., 1997. Sliding mesh simulation of laminar flow in stirred reactors. Transactions of the Institution of Chemical Engineers, 75, 42M4. 

Consider Equations (6-10) that represent the CVD reactor problem. This is a boundary value problem in which the dependent variables are velocities (u,V,W), temperature T, and mass fractions Y. The mathematical software is a stand-alone boundary value solver whose first application was to compute the structure of premixed flames.Subsequently, we have applied it to the simulation of well stirred reactors,and now chemical vapor deposition reactors. The user interface to the mathematical software requires that, given an estimate of the dependent variable vector, the user can return the residuals of the governing equations. That is, for arbitrary values of velocity, temperature, and mass fraction, by how much do the left hand sides of Equations (6-10) differ from zero ... 

This paper deals with the advanced CFD of turbulent stirred vessels. The advances attained in recent years in the field of CFD really matter for the degree of accuracy and confidence at which the performance of stirred reactors and of other operations carried out in stirred vessels can be simulated, as these processes and operations may strongly depend on the details of the physics and chemistry involved. The latter details require the more advanced CFD techniques indeed. 

COSILAB Combustion Simulation Software is a set of commercial software tools for simulating a variety of laminar flames including unstrained, premixed freely propagating flames, unstrained, premixed burner-stabilized flames, strained premixed flames, strained diffusion flames, strained partially premixed flames cylindrical and spherical symmetrical flames. The code can simulate transient spherically expanding and converging flames, droplets and streams of droplets in flames, sprays, tubular flames, combustion and/or evaporation of single spherical drops of liquid fuel, reactions in plug flow and perfectly stirred reactors, and problems of reactive boundary layers, such as open or enclosed jet flames, or flames in a wall boundary layer. The codes were developed from RUN-1DL, described below, and are now maintained and distributed by SoftPredict. Refer to the website http //www.softpredict.com/cms/ softpredict-home.html for more information. 

Fig. 15.9 Steady-state solutions for the benzene mole fraction from the simulation of benzene oxidation near a turning point in a perfectly stirred reactor. Depending on the starting estimates, a number of spurious (nonphysical) solultions may be encountered. The true solution is indicated by the filled circles, while the shaded diamonds indicate (sometimes spurious) solutions that are computed through various continuation sequences.

Simulate the process using a perfectly stirred reactor model. For the nominal processing conditions, plot the O-atom number density as a function of pressure for... 

Aurora, simulates zero-dimensional perfectly stirred reactor problems, including heterogeneous chemistry at the walls. 

The main physicochemical processes in thin-film deposition are chemical reactions in the gas phase and on the film surface and heat-mass transfer processes in the reactor chamber. Laboratory deposition reactors have usually a simple geometry to reduce heat-mass transfer limitations and, hence, to simplify the study of film deposition kinetics and optimize process parameters. In this case, one can use simplified gas-dynamics reactor such as well stirred reactor (WSR), calorimetric bomb reactor (CBR, batch reactor), and plug flow reactor (PFR) models to simulate deposition kinetics and compare theoretical data with experimental results. 

The recent progress in experimental techniques and applications of DNS and LES for turbulent multiphase flows may lead to new insights necessary to develop better computational models to simulate dispersed multiphase flows with wide particle size distribution in turbulent regimes. Until then, the simulations of such complex turbulent multiphase flow processes have to be accompanied by careful validation (to assess errors due to modeling) and error estimation (due to numerical issues) exercise. Applications of these models to simulate multiphase stirred reactors, bubble column reactors and fluidized bed reactors, are discussed in Part IV of this book. 

Flow in baffled stirred reactors has been modeled by employing several different approaches which can be classified into four types, and are shown schematically in Fig. 10.3. Most flow simulations of stirred vessels published before 1995 were based on steady-state analyses (reviewed by Ranade, 1995) using the black box approach. This approach requires boundary conditions (mean velocity and turbulence characteristics) on the impeller swept surface, which need to be determined experimentally. Although this approach is reasonably successful in predicting the flow characteristics in the bulk of the vessel, its usefulness is inherently limited by the availability of data. Extension of such an approach to multiphase flows and to industrial-scale reactors is not feasible because it is virtually impossible to obtain (from experiments) accurate... 

Two-fluid or multifluid models can be extended to simulate not only gas-liquid flows but also any combinations of different phases present in stirred reactors. To simulate gas-liquid-solid, slurry reactors, liquid and solid phases are often lumped together and treated as a slurry phase with effective properties. This approximation is reasonable as long as the solid volume fraction is low ( 1 %). For higher solid loading. 

Perfectly mixed reactors are the key element for conducting chemical processes and in simulation of complex flow systems. Other synonyms are mixed reactor, back mix reactor, an ideal stirred reactor and the CFSTR (constant flow stirred tank reactor). As the name implies, it is a reactor in which the contents are well stirred and uniform throughout. Thus, the exit stream from this reactor has the same composition as the fluid within the reactor. 

Revstedt J, Fuchs L, Tragardh C (1998) Large eddy simulations of the turbulent flow in a stirred reactor. Chem Eng Sci 53 (24) 4041-4053... 

Murthy, B.N., Ghadge, R.S., and Joshi, J.B. (2007), CFD simulations of gas-liquid-solid stirred reactor Prediction of critical impeller speed for solid suspension, Chemical Engineering Science, 62(24) 7184-7195. 

---