Fluidized catalyst beds mass transfer

VI. Heat and Mass Transfer in Fluidized Catalyst Beds. 360... 

First, it will be shown that flow properties of the fluidized catalyst bed (FCB) are clearly different from those of other conventional fluidized beds. The different treatment required is very significant for research and development on fluidized catalytic beds. Next, factors affecting the flow properties are discussed, especially particle size distribution, and also heat and mass transfer, and mixing properties. 

Heat and mass transfer constitute fundamentally important transport properties for design of a fluidized catalyst bed. Intense mixing of emulsion phase with a large heat capacity results in uniform temperature at a level determined by the balance between the rates of heat generation from reaction and heat removal through wall heat transfer, and by the heat capacity of feed gas. However, thermal stability of the dilute phase depends also on the heat-diffusive power of the phase (Section IX). The mechanism by which a reactant gas is transferred from the bubble phase to the emulsion phase is part of the basic information needed to formulate the design equation for the bed (Sections VII-IX). These properties are closely related to the flow behavior of the bed (Sections II-V) and to the bubble dynamics. 

If the same concentration driving force exists in both fixed and fluidized beds, use typical property values to determine the relative rates of mass transfer in these systems. Mass velocities employed for operation of fixed and fluidized beds may be taken as 0.15 and 0.03 g/(cm - s), respectively. Bed void fractions may be taken as 0.30 and 0.80 for the fixed and flnidized beds, respectively. The corresponding catalyst sizes may be taken as 0.5 cm and 0.0063 cm (250 mesh). These nnmbers are chosen so as to favor fixed bed mass transfer. The reacting fluid may be regarded as a gas with a viscosity of 3.30 x 10 g/(cm- s). 

M ass Transfer. Mass transfer in a fluidized bed can occur in several ways. Bed-to-surface mass transfer is important in plating appHcations. Transfer from the soHd surface to the gas phase is important in drying, sublimation, and desorption processes. Mass transfer can be the limiting step in a chemical reaction system. In most instances, gas from bubbles, gas voids, or the conveying gas reacts with a soHd reactant or catalyst. In catalytic systems, the surface area of a catalyst can be enormous. Eor Group A particles, surface areas of 5 to over 1000 m /g are possible. 

Still another advantage of fluidized bed operation is that it leads to more efficient contacting of gas and solid than many competitive reactor designs. Because the catalyst particles employed in fluidized beds have very small dimensions, one is much less likely to encounter mass transfer limitations on reaction rates in these systems than in fixed bed systems. 

One can increase the external mass transfer rate by increasing the flow velocily u over the catalyst pellet. In a slurry this can be accomplished by increasing the stirring, and in a packed bed or fluidized bed it can be increased somewhat by increasing the flow velocity (although with the same catalyst this decreases t, which lowers the conversion). 

Werther, J., Hydrodynamics and mass transfer between the bubble and emulsion phases in fluidised beds of sand and cracking catalyst, in Fluidization (eds. D. Kunii and R. Toei), Engineering Foundation, New York, 93 (1983)... 

A fluidized-bed catalytic reactor system developed by C. E. Lummus (323) offers several advantages over fixed-bed systems in temperature control, heat and mass transfer, and continuity of operation. Higher catalyst activity levels and higher ethylene yields (99% compared to 94—96% with fixed-bed systems) are accomplished by continuous circulation of catalyst between reactor and regenerator for carbon bum-off and continuous replacement of catalyst through attrition. 

The effectiveness of the gas-solid mass transfer in a circulating fluidized bed (see Chapter 10) can be reflected by the contact efficiency, which is a measure of the extent to which the particles are exposed to the gas stream. As noted in Chapter 10, fine particles tend to form clusters, which yield contact resistance of the main gas stream with inner particles in the cluster. The contact efficiency was evaluated by using hot gas as a tracer [Dry et al., 1987] and using the ozone decomposition reaction with iron oxide catalyst as particles [Jiang etal., 1991], It was found that the contact efficiency decreases as the particle concentration in the bed increases. At lower gas velocities, the contact efficiency is lower as a result of lower turbulence levels, allowing a greater extent of aggregate formation. The contact efficiency increases with the gas velocity, but the rate of increase falls with the gas velocity. 

In industrial practice, the laboratory equipment used in chemical synthesis can influence reaction selection. As issues relating to kinetics, mass transfer, heat transfer, and thermodynamics are addressed, reactor design evolves to commercially viable equipment. Often, more than one type of reactor may be suitable for a given reaction. For example, in the partial oxidation of butane to maleic anhydride over a vanadium pyrophosphate catalyst, heat-transfer considerations dictate reactor selection and choices may include fluidized beds or multitubular reactors. Both types of reactors have been commercialized. Often, experience with a particular type of reactor within the organization can play an important part in selection. 

Here c, is the porosity of the catalyst particles, a is the local mass-transfer area per unit of fluidized-bed volume, which can be calculated as... 

The calculated vapor pressure of vanadic acid is a factor of 30 lower than the equilibrium vapor pressure of vanadic acid over pure V2O5. At this vapor pressure of vanadic acid, the transfer of V to trap is rapid while the removal of V by transpiration is negligible. Approximately 64% would be transferred from catalyst to trap, and about 0.05% removed by transpiration. This is rationalized by noting that the velocity of the vanadic acid vapor (calculated from the kinetic theory of gasesX 4.4 x 10" cm s, is four orders of magnitude higher than the superficial velocity of the fluidizing gas. Equation 3a From these calculations it is clear that mass transfer of vapor phase vanadic acid in a fluid bed is sufficient to account for the transport of vanadium from catalyst to trap. 

The results of this work show that even though the vapor pressure of vanadium is low, the transfer velocity of vanadium vapor is high and the rate of mass transfer in a fluidized bed is high. A high rate of vanadium transport to traps and a low rate of vanadium transport by transpiration are consistent with the vapor phase transport model. The vapor pressure of the vanadic acid follows a second order Freundlieh isotherm, which reflects a coverage dependent heat of adsorption. The rate of vanadium transfer from catalyst to trap is only weakly dependent on the number density of the catalyst or trap particles. This lack of dependence suggests that inter-particle collisions are not the dominant mechanism for vanadium transfer. Vanadium mobility in FCCU s is a complex issue dependent on many operating variables. 

In fluidized bed units the plastics are dispersed over the surface of innumerable sand or catalyst particles, greatly facilitating heat and mass transfer. 

The catalyst makes contact with melted MWP. Good contact between plastic particles and the catalyst is one of the key points for process development. Melted plastics can be degraded in a fluidized-bed reactor or a fixed-bed reactor. Since the usage of fixed beds leads to problems of blockage, scale-up to industrial size is not feasible. However the fluidized bed has a number of special advantages for catalytic degradation of plastics, because it is characterized by a good contact between catalyst and plastics as well as an excellent heat and mass transfer [4], In addition to selection of a snitable reactor, the catalyst used is very important in the process. 

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