Liquid backmixing

MV by considering the degree of mixing on the tray. It is obvious that for a small plate with completely backmixed liquid,... 

Gas-Uquid agitated thin film Holdup liquid holdup per total reactor volume 0.0002-0.2 m Liquid loading 0.06-1.25 L/s m of length. Backmixing liquid plug flow or as a series of at least 5 backmix stages energy needed 1 kW/m film surface. Mass transfer controlled reactions. 

Satterfield and Huff (32) have developed a model based on the plug flow of the gas phase and completely backmixed liquid phase and the model equation is based on the summation of mass transfer and reaction resistances. These authors analyzed the data of Schlesinger et al. (35) and the pilot plant scale data of Farley and Ray (36) and concluded that the overall rate is equally Influenced by mass transfer and reaction resistances at normal operating temperatures (around 503 K) and the mass transfer resistance becomes increasingly more important at higher temperatures. ... 

Eq. (60) corresponds with eq. (7) for constant gas flow. As usual, it is now necessary to eliminate Cj i by accounting for the reaction rate in the backmixed phase. Assuming m-th order reaction of A in the backmixed liquid phase one obtains... 

Fig. 4 Effect of gas phase mixing on conversion and absorption rate for first order reaction - Comparison of models <11> and <13> both with backmixed liquid phase...

Liquid Dispersion Spray columns are used with slurries or when the reaction product is a solid. The absorption of SO9 by a hme slurry is an example. In the treatment of phosphate rock with sulfuric acid, offgases contain HF and SiF4. In a spray column with water, solid particles of fluorosilic acid are formed but do not harm the spray operation. The coefficient /cl in spray columns is about the same as in packed columns, but the spray interfacial area is much lower. Considerable backmixing of the gas also takes place, which helps to make the spray volumetri-caUy inefficient. Deentrainment at the outlet usually is needed. 

It seems probable that a fruitful approach to a simplified, general description of gas-liquid-particle operation can be based upon the film (or boundary-resistance) theory of transport processes in combination with theories of backmixing or axial diffusion. Most previously described models of gas-liquid-particle operation are of this type, and practically all experimental data reported in the literature are correlated in terms of such conventional chemical engineering concepts. In view of the so far rather limited success of more advanced concepts (such as those based on turbulence theory) for even the description of single-phase and two-phase chemical engineering systems, it appears unlikely that they should, in the near future, become of great practical importance in the description of the considerably more complex three-phase systems that are the subject of the present review. 

The kinetic models for the gas phase polymerization of propylene in semibatch and continuous backmix reactors are based on the respective proven models for hexane slurry polymerization ( ). They are also very similar to the models for bulk polymerization. The primary difference between them lies in the substitution of the appropriate gas phase correlations and parameters for those pertaining to the liquid phase. 

Scientists of the Institute of Science and Technology of UMIST (Manchester, UK) and collaborating laboratories have developed a network-of-zones model (Mann et al, 1981 Mann and Knysh, 1984 Mann and El-Hamouz, 1992, 1995 Mann et al, 1995 Nienow et al, 1992 Togatorop et al, 1994 and Wang and Mann, 1990, 1992). According to this model a reaction volume is represented by an assembly of backmixed zones, each of equal volume. The liquid... 

The PFR model is frequently used for a reactor in which the reacting system (gas or liquid) flows at relatively high velocity (high Re, to approach PF) through an otherwise empty vessel or one that may be packed with solid particles. There is no device, such as a stirrer, to promote backmixing. The reactor may be used in large-scale operation... 

Assuming the liquid phases remain immiscible, the modelling approach for multicomponent systems remains the same, except that it is now necessary to write additional component balance equations for each of the solutes present, as for the multistage extraction cascade with backmixing in Section 3.2.2. Thus for component j, the component balance equations become... 

In packed-bed reactors, the catalyst is fully wetted, whereas the heat and mass transfer efficiency is higher than that observed in trickle-bed reactors. However, low operation efficiency may appear due to backmixing of the liquid phase. Moreover, high liquid-phase residence times can result in the occurrence of homogeneous side reactions. 

Continuous flow of both phases in upflow and complete mixing of phases For packed bubble columns (upflow of both gas and liquid phases), under the assumption of complete mixed flow, the backmixing model of Ramachandran and Chaudhari (1980) is applicable. The relevant equations are presented in Section 3.5.1 for the continuous flow of gas and slurry phases in complete mixed-flow conditions (slurry CSTR reactor). 

Concerning packed bubble bed reactors, the evaluation of the Peclet number of the liquid-phase is important in order to decide if we have to use a plug- or backmixed-flow model. The liquid-phase can be considered well mixed if (Ramachandran and Chaudhari, 1980)... 

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