Mass transfer/reaction

Fluid mixing is a unit operation carried out to homogenize fluids in terms of concentration of components, physical properties, and temperature, and create dispersions of mutually insoluble phases. It is frequently encountered in the process industry using various physical operations and mass-transfer/reaction systems (Table 1). These industries include petroleum (qv), chemical, food, pharmaceutical, paper (qv), and mining. The fundamental mechanism of this most common industrial operation involves physical movement of material between various parts of the whole mass (see Supplement). This is achieved by transmitting mechanical energy to force the fluid motion. 

Mixing class Physical Mass-transfer/reaction... 

Chemical vapor deposition processes are complex. Chemical thermodynamics, mass transfer, reaction kinetics and crystal growth all play important roles. Equilibrium thermodynamic analysis is the first step in understanding any CVD process. Thermodynamic calculations are useful in predicting limiting deposition rates and condensed phases in the systems which can deposit under the limiting equilibrium state. These calculations are made for CVD of titanium - - and tantalum diborides, but in dynamic CVD systems equilibrium is rarely achieved and kinetic factors often govern the deposition rate behavior. 

Smaller bore diameters naturally produce slugs of smaller diameter [31,97]. Typically, a smaller length can also be generated thereby. As a consequence, internal circulation in the slug and specific interface between the slugs are increased. It is assumed that the impact of the increase in internal circulation on mass transfer/ reaction processing is generally more dominant. 

A2 Sauter Mean (SMD) (Volume-Surface) 3 2 Z A A3 Zaa2 Mass Transfer, Reaction... 

At the air-water interface, water molecules are constantly evaporating and condensing in a closed container. In an open container, water molecules at the surface will desorb and diffuse into the gas phase. It is therefore important to determine the effect of a monomolecular film of amphiphiles at the interface. The measurement of the evaporation of water through monolayer films was found to be of considerable interest in the study of methods for controlling evaporation from great lakes. Many important atmospheric reactions involve interfacial interactions of gas molecules (oxygen and different pollutants) with aqueous droplets of clouds and fog as well as ocean surfaces. The presence of monolayer films would thus have an appreciable effect on such mass transfer reactions. 

Plug Flow G/Plug Flow L—Mass Transfer + Reaction in a Countercurrent Tower... 

Note that these are the conventional CSTR equations in a and p phases with the mass transfer reactions between phases added. 

A commonly used mass transfer reaction model is presented in Figure 8.1a, where the reaction occurs in the bulk aqueous phase [35, 47]. It is assumed that the substrate dissolved in the organic phase diffuses into the aqueous phase, reaching equilibrium. In the absence of reaction, once equilibrium is achieved, apparent mass transfer ceases. Given the presence of active enzyme, depletion of substrate in the aqueous phase occurs, and the system moves into a new equilibrium. Thus, the overall reaction rate depends both on reaction and mass transfer. 

The model provides a good approach for the biotransformation system and highlights the main parameters involved. However, prediction of mass transfer effects on the outcome of the process, through evaluation of changes in the mass transfer coefficients, is rather difficult. A similar mass transfer reaction model, but based on the two-film model for mass transfer for a transformation occurring in the bulk aqueous phase as shown in Figure 8.3, could prove quite useful. Each of the films presents a resistance to mass transfer, but concentrations in the two fluids are in equilibrium at the interface, an assumption that holds provided surfactants do not accumulate at the interface and mass transfer rates are extremely high [36]. 

A stand-alone mixer requires the mass transfer/reaction to be completed within the mixer. If the gas flow rate matches the stoichiometry of the liquid phase, all the gas should be dissolved and reacted at the end of the mixer. This generally involves very high volumetric ratios between gas and liquid. If there is excess gas, there will be some gas at the mixer outlet, which needs to be separated. 

Despite the clear importance of reactive absorption, its behavior is still not properly understood. This can be attributed to a very complex combination of process thermodynamics and kinetics, with intricate reaction schemes including ionic species, reaction rates varying over a wide range, and complex mass transfer-reaction coupling. As compared to distillation, reactive absorption is a fully rate-controlled process and it occurs definitely far from the equilibrium state. Therefore, both practitioners and theoreticians are highly interested to establish a proper understanding and description of this process. 

E. Y. Kenig, Mass transfer-reaction coupling in two-phase multicomponent fluid systems, Chem. Eng. J., 1995, 57, 189-204. 

For the system (2.36), in the limit e —> 0, the term (l/sjkfx) becomes indeterminate. For rate-based chemical and physical process models, this allows a physical interpretation in the limit when the large parameters in the rate expressions approach infinity, the fast heat and mass transfer, reactions, etc., approach the quasi-steady-state conditions of phase and/or reaction equilibrium (specified by k(x) = 0). In this case, the rates of the fast phenomena, as given by the explicit rate expressions, become indeterminate (but, generally, remain different from zero i.e., the fast reactions and heat and mass transfer do still occur). 

Under intrinsic-kinetic conditions the carbon number distribution of products from a reduced fused magnetite catalyst Is not significantly affected by wide variations In H, and CO concentration and mass-transfer resistances have no noticeable effect, as would be expected. To the extent that other selectlvltles, such as oxygenate product composition, are governed by H and CO concentrations In the liquid, we would similarly expect to observe effects caused by mass transfer, although this was not done here. Likewise with other catalysts, such as cobalt, which appear to be more sensitive to reaction conditions and to secondary reactions, more marked effects from significant mass transfer reactions are anticipated. 

Convection-Mass-Transfer Reaction in the Bulk 12.4.3.1 Bulk Gas... 

A set of five programs known as The Geochemist s Workbench or GWB was developed by Bethke (1994) with a wide range of capabilities similar to EQ3/6 and PHREEQC v. 2. GWB performs speciation, mass transfer, reaction-path calculations, isotopic calculations, temperamre dependence for 0-300 °C, independent redox calculations, and sorption calculations. Several electrolyte databases are available including ion association with Debye-Huckel activity coefficients, the Pitzer formulation, the Harvie-M0ller-Weare formulation, and a... 

Surface reactions options for rate controlling steps/UD Multiphase reactive flows mass transfer/reactions in all phases ... 

The first purpose of sample preparation is to stop the reaction in order to determine the current state of the reaction. Dilution of a small aliquot into a large volume of solvent, e.g., 20 pi of a reaction diluted into 10 ml of HPLC mobile phase, effectively stops most reactions. Dilution is a necessary part of sample procedure for todays sensitive analytical techniques, such as GC and HPLC. For reactions run at high temperature, cooling may slow down the reaction and effectively stop it. More reactive aliquots may be quenched prior to assaying. In heterogeneous reactions that are limited by mass transfer, reaction may be stopped by stopping agitation. The reactions of most samples are stopped by dilution into another solvent. 

Mineralogical information is recommended. Guesses of possible mass transfer reactions are required. 

A mathematical model is essential for process optimization and scale up. For solid-state fermentation, setting up a mathematical model is a much more difficult task than for the submerged culture process because of the difficulties in parameter measurement and the complexity in mass transfer reaction interaction. 

Design based on mass transfer. Reactions are usually very rapid and design is based on mixing to distribute the reactant. For most precipitation reagents, allow 5-min residence time. If secondary reagents are needed to change the oxidation state of the target species before precipitation, then example residence times are... 

Figure 2.6. Schematic representation of the inverse mass balance model. If we know that the spring at the foothill is evolved from rain water, possible mass transfer reactions can be modeled from the mass balance principle.

Inverse mass balance modeling here only employs the mass balance principle thermodynamics and equilibrium are not considered. Inverse models are usually nonunique. A number of combinations of mass transfer reactions can produce the same observed concentration changes along the flow path. Mass transfer reactions here refer to the reactions that result in the mass transfer between two or more phases, such as the dissolution of solid and gas or precipitation of solids. Chapter 9 describes the details of the models and shows a few examples. 

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