Gas-flowing solids-fixed bed contactors

In this type of equipment, fine solid particles are introduced at the top of the column containing a packed bed, while gas can be introduced either at the bottom for countercurrent contacting or at the top when cocurrent operation is desired. In the literature, such columns are named gas-flowing solids-fixed bed contactors, or raining packed bed contactors, or solids trickle flow contactors, while the titles in the Russian literature were more descriptive. 

FIGU RE 22.2 Typical results for pressure drop in gas-flowing solids-fixed bed contactors. Pressure drop as a function of superficial gas velocity for different values of flowing solids flow rates. Different s3Tnbols correspond to different flowing solids mass fluxes in the range 0-6.13 kg/m sec. Ap/L increases with S, square symbols are for S = 0. (From Roes, A.W.M. and van Swaaij, W.P.M., Chem. Eng. J., 17, 81, 1979. With permission.)... 

Studies of the Hydrodynamics of Gas-Flowing Solids-Fixed Bed Contactors... 

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]. 

The high values of the heat transfer coefficients are one of the most favorable advantages of gas-flowing solids-fixed bed contactors. High heat transfer rates between fine solids and gas, along with low pressure drop in the system, make this type of equipment very attractive for applications. This has been estabhshed in industrial practice as raining packed bed exchangers [31]. 

The essential reasons for their significant thermal performance are good radial distribution of gas and flowing solids and large interfacial area available for heat transfer. Thermal properties have been smdied experimentally [7,32-34], and theoretical models have been proposed [7,32]. The heat transfer properties of gas-flowing solids-fixed bed contactors not only depend on gas and solids flow rates and type of packing, but also on the portion of static holdup and axial dispersion. 

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 contactor was patented as early as 1948 [1]. The first industrial realization occurred in France in 1965, named the raining packed bed exchanger [31] for heat recuperation process. Since then, it has been efficiently exploited. Potential applications are gas purification, adsorption processes, drying, etc. Another interesting concept would be the use of gas-flowing solids-fixed bed catalytic reactors for the equilibrium reactions with separation of products in situ. 

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. 

Gas-flowing solids-fixed bed contactors can be used either as two-phase continuous contactors between flowing solids and gas where the fixed bed structure can be specially designed for distribution of both flowing phases or as three-phase contactors where the fixed bed has an active role (e.g., catalyst bed). 

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