Contactors solid-fluid

Coupling two operations like membrane separation and a catalytic reaction or adsorption in a given process of synthesis, purification, or decontamination of effluents is intrinsically interesting from a general technical-economical point of view. Ceramic membranes are ideal solid-fluid contactors, which can be efficiently used to couple separation and heterogeneous catalysis for membrane reactor applications. ... 

Class 1 equipment are also called column-type equipment. Under this category, there are the various multiphase contactors. Gas-liquid contactors include bubble columns, packed bubble columns, internal-loop and external-loop air-lift reactors, sectionalized bubble columns, plate columns, and others. Solid-fluid (liquid or gas) contactors include static mixers, fixed beds, expanded beds, fluidized beds, transport reactors or contactors, and so forth. For instance, fixed-bed geometry is used in unit operations such as ion exchange, adsorptive and chromatographic separations, and drying and in catalytic reactors. Liquid-liquid contactors include spray columns, packed extraction... 

One fluid phase in contact with one solid phase liquid-solid gas/vapor-solid supercritical fluid-solid (solid-fluid phase membrane contactors) (Fig. 26.2). 

Figure 26.2 Membrane contactor for solid-fluid phase contacting.

The interfacial area in the contactor, which is directly related to the solids hold-up, strongly influences the mass transfer rate. To maximise the overall mass-transfer rate per unit volume of equipment, a high solids hold-up is necessary. On the other hand, the solids hold-up also influences the pressure drop over the contactor. The pressure drop has a hydrostatic and a dynamic component, both of which rise with increased solids hold-up. Since the adsorbent consists of extremely small particles, fluid friction between liquid and solids may lead to a relatively high dynamic pressure drop. The hydrostatic pressure drop is attributable to the density difference between the suspension in the contact zone and in the liquid. 

In many multiphase (gas-liquid, gas-solid, liquid-liquid and gas-liquid-solid) contactors, a large degree of circulation of both discrete and continuous phases occurs. This circulation causes a good degree of mixing and enhances heat and mass transfer between fluid and walls. The degree of circulation depends on a number of parameters such as the size of equipment, the nature of the phases involved, velocities of various phases, nature of the internals within the equipment and many others. 

Figure 32. The Himsley Continuous Fluid-Solid contactor.t ...

Tables 2.4 and 2.5 compare the features of conventional and modified fluid bed and spouted bed dryers, respectively. In order to choose between them, one must know and compare the specific merits and demerits of each type of gas-solids contactor. With the new devices often the data available in the literature is obtained at laboratory scale—only in a few cases it may be pilot scale. The scale-up is, therefore, difficult and uncertain. One must objectively evaluate the potential offered by the new technology and, if justified, carry out a systematic pilot scale study. Often it may be possible to scale up the heat and mass transfer characteristics. However, the quality of the dried product is difficult to predict actual experimental testing is therefore a necessity.

Most of the fluid dynamic studies of gas-flowing solids-fixed bed contactors were devoted to countercurrent flow systems, because of the higher efficiency of countercurrent operations for most of the processes when compared with cocurrent operations. However, there is an upper limit for gas flow rate in countercurrent systems, due to flooding. Hence, the cocurrent operation system is an interesting alternative for higher gas flow rates, particularly for very small particles. Further, in some of the proposed applications, cocurrent contacting is a desirable flow pattern [22]. 

Mass transfer rates in gas-flowing solids-fixed bed contactors are expected to be high, according to fluid dynamics and heat transfer behavior. Somewhat lower values of mass transfer coefficients than those expected were reported in the literature [6,35-37]. The reasons for that are the effects of segregation as well as strong influence of axial backmixing. Apart from this, mass transfer rates depend on size and structure (porosity) of flowing solids [36]. 

Gas-flowing solids-fixed bed contactors, according to their mass transfer and fluid dynamics properties, are suitable for various environmental appUcations, which involve adsorption of pollutants. Several investigations [6,10,36,37,41] have been carried out at pilot-plant level, and the results were promising for the design of industrial equipment. In the presented works [6,10,36,37,41], the adsorption process is followed by the chemical reaction on the flowing solids phase. 

Benah, M., Shakourzadeh-Bolouri, K., and Large, U., Hydrodynamic characterization of dilute suspensions in the gas-solid-solid packed contactors. Proceedings of the Seventh Found. Conf. Fluid., 651-658, 1992. 

Similarly to partially overlapping channels, microchannels with mesh contactors (Figure 7.2h) are used to create the partial contact of fluids. The advantage of these contactors is that both modes of operation, cocurrent and countercurrent, can be apphed. Besides, the flow is stabilized because of the solid support between two fluids. The solid contactors are porous membrane [9, 10] and metal sheets with sieve-like structure [11]. Similarly to parallel flow, the mass transfer in both cases is only by diffusion and the flow is under laminar flow regime dominated by capillary forces. The membrane contactor has the advantage of being flexible with respect to the ratio of two fluids. In addition to flow velocities, the mass transfer is a function of membrane porosity and thickness. In another type of microextractor, two microchaimels are separated by a sieve-like wall architecture to achieve the separation of two continuous phases. However, the hydrodynamics in both types of contactors is more complex because of interfadal support and bursting of fluid... 

Panneerselvam R, Savithri S, Surender GD. (2009) Computational fluid dynamics simulation of solid suspension in a gas-liquid-solid mechanically agitated contactor. Ind. Eng. Chem. Res., 48 1608-1620. 

The solid supported extractants can be employed in fixed bed contactors to extract metal ions from solutions. Other geometries include slurry extractors and moving bed adsorbers. We consider a fixed bed geometry. In this case the following mass transfer processes may be present (1) interpellet mass transfer, which refers to the diffusion and mixing of metal ion in fluid occupying the spaces between pellets (2) interphase mass transfer, which is the transfer of metal ion across the fluid peUet interface and (3) intraparticle mass transfer, which is the diffusion of metal ions in... 

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