Mass Transfer and Mixing

For predicting mass transfer in Taylor flow, gas-liquid and liquid-wall mass transfer coefficients are required. Mass transfer coefficients from gas to liquid were found experimentally to be in the range 0.1-0.8 s and correlations have been 

A model must be introduced to simulate fast chemical reactions, for example, flamelet, or turbulent mixer model (TMM), presumed mapping. Rodney Eox describes many proposed models in his book [23]. Many of these use a probability density function to describe the concentration variations. One model that gives reasonably good results for a wide range of non-premixed reactions is the TMM model by Baldyga and Bourne [24]. In this model, the variance of the concentration fluctuations is separated into three scales corresponding to large, intermediate, and small turbulent eddies. 

An estimation of the importance of mixing on reaction rate can be done by calculating the Dahmkohler number Time for mixing 

According to the TMM model, the time constant for mixing on the large turbulent scale is on the order of r = O.Sk/s, that is, ms in a stirred tank reactor, and the mixing on intermediate scale is on the order of r = 17.25y u/e, that is, R 20ms. 

The smallest scale will have an effect only for very large Sc numbers. If a noticeable conversion can be observed within 25 ms, the mixing will have an effect on reaction rate, that is, the reaction is mixing independent only if Da 1. 

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It is well understood that mixing and mass transfer are affected by agitation and aeration rates. Typically, in an agitated vessel, the working volume is about 75% of nominal CSTR volume. 

The difficulty disappears when the mixing and mass transfer steps are fast compared with the reaction steps. The contents of the reactor remain perfectly mixed even while new ingredients are being added. Compositions and reaction rates will be spatially uniform, and a flow term is simply added to the mass balance. Instead of Equation (2.30), we write... 

C >70% accumulation of energy Very fast <1 s mixing controlled High quality mixing and mass transfer. High heat transfer area gives temperature control. Intensified and/or structured reactors. 

Bubble columns and mechanically stirred reactors are the most common reactor types for slurry systems in laboratories, but they have many disadvantages from an industrialization perspective. Mechanically stirred reactors usually used for laboratorial studies are difficult to scale-up. In order to achieve good mixing and mass transfer between the gas and slurry phases, bubble column must be operated at a high space velocity, which leads to a relative low one-through conversion of the syngas. 

Figure 5.126. Gas phase mixing and mass transfer for full circulation case.

GAS-LIQUID MIXING AND MASS TRANSFER IN BUBBLE COLUMN-TYPE MIXING TANK OPERATION EXAMPLE FOR OXYGEN CONSTANT VL1=300, VL2=100,VL3=600 L... 

GAS-LIQUID MIXING AND MASS TRANSFER WITH FULL... 

GASLIQ1 and GASLIQ2 - Gas-Liquid Mixing and Mass Transfer in a Stirred Tank... 

Gas-Liquid Mixing and Mass Transfer 385 Dynamics of a Shell-and-Tube Heat Exchanger 511 Semi-Batch Manufacture of Hexamethylene-triamine 353... 

The technology can be tuned to indnce large-amplitnde sonic pulsations inside combustion process units such as boilers and incinerators. These pulsations increase heat release, mixing, and mass-transfer rates in the combustion process, resulting in faster and more complete combnstion. 

Conventional Ni-based catalysts still dominate in SR applications however, ceria-supported noble metal catalysts have also attracted interest reeently. The study of Rh for both POX and ATR has increased sinee Rh is in general more active for reforming and is less prone to form carbon. H2 and CO selectivities in Rh-based catalysts have been shown to be affected by catalyst geometry. This indicates that feed mixing and mass transfer can play an important role. 

In many catalytic reactions, solid, liquid, and gas phases are involved, and the phase behavior often has a strong influence on mixing and mass transfer and consequently on the catalytic performance. Supercritical fluids, especially supercritical CO2, have gained considerable attention as environmentally benign solvents (e.g., (94y). The combined use of in situ transmission and ATR-IR spectroscopy together with video monitoring is a promising approach for elucidation of the behavior of a... 

Higler AP, Krishna R, Ellenberger J, Taylor R. Countercurrent operation of a structured catalytically packed-bed reactor liquid-phase mixing and mass transfer. Chem Eng Sci 1999 54 5145-5152. 

Additional information on hydrodynamics of bubble columns and slurry bubble columns can be obtained from Deckwer (Bubble Column Reactors, Wiley, 1992), Nigam and Schumpe (Three-Phase Sparged Reactors, Gordon and Breach, 1996), Ramachandran and Chaudhari (Three-Phase Catalytic Reactors, Gordon and Breach, 1983), and Gianetto and Silveston (Multiphase Chemical Reactors, Hemisphere, 1986). Computational fluid mechanics approaches have also been recently used to estimate mixing and mass-transfer parameters [e.g., see Gupta et al., Chem. Eng. Sci. 56(3) 1117-1125 (2001)]. 

Here we examine commonly used mechanically agitated industrial liquid-liquid contactors. Table XVII describes mixing and mass transfer parameters for mechanically agitated liquid-liquid contactors. The most commonly... 

Typical Values of Mixing and Mass-Transfer Parameters in Mechanically Agitated Contactors (Reprinted from Doraiswamy and Sharma. 1984, courtesy of Marcel Dekker, Inc.)... 

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