Interaction between gas and liquid

Lane, G. L., Schwarz, M. P., and Evans, G. M., Modelling of the Interaction Between Gas and Liquid in Stirred Vessels . Proceedings of the 10th European Conference on Mixing, Delft, Netherlands, 197-204 (2000). 

The reduced interaction between gas and liquid during annular flow in the internally finned channel as compared with two-phase flow in a packed bed not only affects transfer... 

As previously mentioned, the mass transfer from the gas to the active sites of the catalyst involves several steps. For trickle bed operation, in which the interaction between gas and liquid is limited, the values for kg and are of the same... 

An alternative approach [11], based on the flow resistance of gas phase, has been proposed for the regime of low interaction between gas and liquid. In this case, a two-phase friction factor, which allows for the increase of liquid holdup by increasing liquid flow rate, is defined and experimental data are correlated according to an Ergun-type equation as ... 

The interaction between gas and liquid is limited at low Re numbers. Neither holdup nor dispersion is strongly affected by the gas flow rate, and molecular diffusion coefficient has a rather small effect on the particle Peclet number [20]. 

In this type of reactor, gas superficial velocity is of the order of 0.1-0.3 m/s which is low enough to avoid mechanical interactions between gas and liquid. The velocity of the liquid, in the range 1 - 8.10 m/s is still low, but sufficient to guarantee satisfactory external wetting of the catalyst particles. Table 1 shows advantages and disadvantages of Fixed Bed Multiphase Reactors. Table 2 shows the characteristic parameters of TBRs compared to the two other important multiphase reactors Stirred Slurry and Flooded Fixed Bed Reactor. 

The application of the effective pressure difference for superheated liquids is shown in Fig. 16.27. For small and medium superheat the discharge coefficient shows similar values. Differences are found for small liquid pressures and high liquid temperature, which promote high vapor contents for which interaction between gas and liquid phase cannot be neglected. 

At the end of this discussion of this figure, we can say that thermodynamics (Chapter 14) suggests an explanation of the hysteresis loop in the adsorption-desorption curves, but gives no suggestions about the offset between the first and second adsorption curves. I end this section on adsorption with this unexplained result and the lame explanation of it to remind the reader that the interactions between gas and liquid molecules and heterogeneous solid surfaces is not nearly as simple or as well understood as the interactions between liquids, gases and homogeneous solids. 

The results of the last section showed that, for any macroscopic container at normal pressures, it is not reasonable to conclude that the molecules proceed from wall to wall without interruption. However, if the interaction potential energy between molecules at their mean separation is small compared to the kinetic energy, the speed distribution and the average concentration of gas molecules is about the same everywhere in the container. In this limit, the only real effect of collisions is the excluded volume occupied by the molecule, which effectively shrinks the size of the container. At 1 atm, only about 1/1000 of the space is occupied (remember the density ratio between gas and liquid), so each additional molecule sees only 99.9% of the container as free space. On the other hand, if the attractive part of the interaction potential cannot be totally neglected, the molecules which are very near the wall will be pulled slightly away from the wall by the other molecules. This tends to decrease the pressure. 

The remainder of the chapter focuses on the actual spray modeling. The exposition is primarily done for the RANS method, but with the indicated modifications, the methodology also applies to LES. The liquid phase is described by means of a probability density function (PDF). The various submodels needed to determine this PDF are derived from drop-drop and drop-gas interactions. These submodels include drop collisions, drop deformation, and drop breakup, as well as drop drag, drop evaporation, and chemical reactions. Also, the interaction between gas phase, liquid phase, turbulence, and chemistry is examined in some detail. Further, a discussion of the boundary conditions is given, in particular, a description of the wall functions used for the simulations of the boundary layers and the heat transfer between the gas and its confining walls. 

There are many techniques for probing the chemical and physical properties of a solid surface to predict the tending of organic polymers to solid surfaces. The electronic structure of solid surfaces has been studied by measuring the thermodynamic interaction of the solid surface with simple liquids of known molecular structure. Experimental techniques for measuring the thermocfynamic interaction between solid and liquid include contact angle measurement, calorimetry, and gas chromatography. Some of these techniques are discussed below. Specific techniques related to characterization of carbon fiber surfaces are also discussed. 

Fig. 1 Schematic representation of columns and gas connections for studying (a) diffusion coefficients in binary gas mixtures, (b) interaction between gases and liquids, and (c) interaction between gases and solids.

The minimum fluidization velocity can also be effectively described based on the gas-perturbed liquid model (Zhang et al., 1998a). The model assumes that the solid particles in the bed are fully supported by the liquid and there is little direct interaction between gas and solid phases. In this model, the gas phase plays a simple role in the bed by occupying space with particles fluidized by the liquid. The minimum fluidization velocity along with minimum liquid velocity for particle transport for three-phase fluidized beds of relatively large particles (dp > 1mm) (Zhang et al., 1998b) can... 

An alternative approach based on gas phase flow resistence (Specchia and Baldi [78]) is valid for low interaction regimes between gas and liquid. A two phase friction factor which allpws for an increase in liquid hold up with liquid rate, is defined according to an Ergun-type equation ... 

The rapid and reversible formation of complexes between some metal ions and organic compounds that can function as electron donors can be used to adjust retention and selectivity in gas and liquid chromatography. Such coordinative interactions are very sensitive to subtle differences in the composition or stereochemistry of the donor ligand, owing to the sensitivity of the chemical bond towards electronic, steric and strain effects. A number of difficult to separate mixtures of stereoisomers and isotopomers have been separated by complexation chromatography. 

Another important effect observed when reactions take place in the liquid phase is associated with the solvation of the reactants. Theoretical comparison showed that the collision frequencies of the species in the gas and liquid phases are different, which is due to the difference between the free volumes. In the gas phase, the free volume is virtually equal to the volume occupied by the gas species (FfwT), while in the liquid phase, it is much smaller than the volume of the liquid species (V < V). Since the motion and collision of the species occur in the free volume, the collision frequency in the liquid is higher than in the gas by the amount (V/Vf)U3 [32,33]. The activation energies for the reactions of radicals and atoms with hydrocarbon C—H bonds in the gas and the liquid phases are virtually identical, and that in the liquid is independent of the solvent polarity. This also applies to the parameter bre, which can be seen from the following examples referring to the interaction of the hydroxyl radical with hydrocarbons [30] ... 

Sander, R. Modeling atmospheric chemistry Interactions between gas-phase species and liquid cloud/aerosol particles, Surv. Geophys., 20, 1-31, 1999. 

Mass transfer (the C term), which involves collisions and interactions between molecules, applies differently to both packed and capillary columns. Packed columns are mostly filled with stationary phase so liquid phase diffusion dominates. The mass transfer is minimized by using a small mass of low-viscosity liquid phase. Capillary columns are mostly filled with mobile phase, so mass transfer is important in both the gas and liquid phases. A small mass of low-viscosity liquid phase combined with a low-molecular weight carrier gas will minimize this term. 

The quantities that best represent a particular property can often be rationalized on the basis of physical intuition. For example, those that reflect interactions between like molecules, such as heats of sublimation and vaporization, can be expressed well in terms of molecular surface area and the product vofot. A large value for this product means that each molecule has both significantly positive and significantly negative surface potentials, which is needed to ensure strongly attractive inter-molecular interactions, with consequently higher energy requirements for the solid —> gas and liquid —> gas transitions. 

For gas-liquid combinations with relatively small uptake coefficients ( 10 4-10-7), longer interaction times between the gas and liquid are needed than can be obtained with the falling-droplet apparatus. These are provided in a bubble apparatus, a typical example of which is shown in Fig. 5.24. The gas of interest as a mixture with an inert carrier gas is introduced as a stream of bubbles into the liquid of interest. The interaction time is varied by moving the gas injector relative to the surface. The composition of the gas exiting the top of the liquid is measured as a function of the interaction time (typically 0.1-1 s), e.g., by mass spectrometry. The interaction time is limited by the depth in the liquid at which the bubbles are injected and their buoyancy. Longer interaction times and better control over them have been achieved using a modified apparatus in which the bubbles are generated and transported horizontally (Swartz et a.l., 1997). 

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