Fluidized catalyst beds dynamics

Currently available data for the flow properties of the fluidized catalyst bed are fragmentary, since the local motion of the emulsion phase is diflicult to measure experimentally. Therefore, it is useful to clarify the flow properties of the bed in terms of our knowledge of bubble columns. First, the fluid-dynamic properties of the bubble columns will be explained then, the available data will be adapted to apply to fluid catalyst beds. The reader will be able to picture an emulsion phase of carefully prepared catalyst particles operating in intense turbulence for fluidized beds under conditions of practical interest. This turbulence distinguishes the flow properties of fluid catalyst beds from those of widely studied teeter beds. 

It is desirable to establish a phenomenologically sound basis by relating Pt to well-established fluid-dynamic concepts, since we need to know the possibility of scale-up or of applying the concept of Pt to a fluidized catalyst bed, as discussed in what follows (M27). 

Longitudinal dispersion in the continuous phase (the liquid phase for a bubble column, and the emulsion phase for a fluidized catalyst bed) is closely related to flow properties of the equipment. Here, we wish to describe the longitudinal dispersion phenomena in terms of the fluid-dynamic properties of the equipment. The prime purpose is to test whether the fluid-dynamic analysis developed earlier is sound, but lon-... 

III,D,5). In this chapter, the equation is further examined in relation to bed performance, since the turbulence properties of the bed result from interaction between bubbles and the continuous phase. As shown in Fig. 34, the mean slip velocity of bubbles in a fluidized catalyst bed of good fluidity is essentially the same as that for a bubble column when Uq > 20 cm/sec. A criterion under which bubble size approaches a dynamic equilibrium is obviously needed for predicting or evaluating the performance of scaled-up beds. 

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. 

Luss and Amundson (1968) have studied the dynamics of catalytic fluidized beds. The system is a good example of a stiff set of differential equations. Catalytic fluidized beds are utilized for a variety of reactions such as oxidation of naphthalene and ethylene and the production of alkyl chlorides. A batch fluidization reactor is usually built as a cylindrical shell with a support for the catalyst bed. The reactants enter from the bottom through a cone and cause the catalyst particles to be fluidized in the reactor. The reactants leave through a cyclone in which the entrained solids are separated and returned to the bed. 

A theory has been developed which translates observed coke-conversion selectivity, or dynamic activity, from widely-used MAT or fixed fluidized bed laboratory catalyst characterization tests to steady state risers. The analysis accounts for nonsteady state reactor operation and poor gas-phase hydrodynamics typical of small fluid bed reactors as well as the nonisothermal nature of the MAT test. Variations in catalyst type (e.g. REY versus USY) are accounted for by postulating different coke deactivation rates, activation energies and heats of reaction. For accurate translation, these parameters must be determined from independent experiments. 

Equipment. A vertical fixed-bed reactor, made of a 0.168 m I.D. and 0.5 m long 316 stainless steel tube with an axial thermowell, was used. The amounts of catalyst used for the steady state and dynamic experiments were 6.35 and 18.69 g, respectively. The reactor tube was heated by a fluidized bed sand bath. The reaction gases, O2 and CO, and the diluent, He, were metered through rotameters qnd mixed prior to their entry to the reactor. The mixing junction was designed such that either of the reaction gases or CO2 could be introduced or removed from the stream to simulate a step increase or decrease of the component in question. The effluent from the reactor was analyzed by gas chromatography in 4 minutes. 

Solids of group A have small particle diameters (% 0.1 mm) or low bulk densities this class includes catalysts used, for example, in the fluidized-bed catalytic cracker. As the gas velocity u increases beyond the minimum fluidization point, the bed of such a solid first expands uniformly until bubble formation sets in at u = //mb > mr. The bubbles grow by coalescence but break up again after passing a certain size. At a considerable height above the gas distributor grid, a dynamic equilibrium is formed between bubble growth... 

In some situations the dynamics of the cooling system may be such that effective temperature control cannot be accomplished by manipulation of the coolant side. This could be the situation for fluidized beds using air coolers to cool the recirculating gases or for jacketed CSTRs with thick reactor walls. The solution to this problem is to balance the rate of heat generation with the net rate of removal by adjusting a reactant concentration or the catalyst flow. Such a scheme is shown in Fig. 4.24. 

Consider the dynamics of a catalytic fluidized bed in which an irreversible gas phase reaction takes place (Aiken and Lapidus, 1974 [17] Cutlip and Shacham, 1999). [9] Partial pressure of reactant in fluid (P), temperature of reactant in fluid (T), partial pressure of reactant at the catalyst surface (Pp) and partial pressure of reacfanf at the catalyst surface (Tp) are governed by the following equations ... 

Dynamic Model for the Catalyst Pellet Mathematical Modeling and Simulation of Fluidized-Bed Reactors... 

Rahimpour MR, Bayat M, Rahmani F Dynamic simulation of a cascade fluidized-bed membrane reactor in the presence of long-term catalyst deactivation for methanol synthesis, Chem Eng Set 65 4239—4249, 2010. 

The advantages in fluidized-bed operation for heat management and for maintenance of constant catafyst activity have already been noted. Other advantages include higher yield, quality, lower durene, and potentially lower investment eosts. The fluidized-bed operation requires catalysts with low attrition properties. In the design of a fluidized-bed reaetor system, particular attention must be given to the reactor fluid dynamics to ensure complete methanol conversion. This is critical to avoid the need for additional distillation facilities to reeover unreaeted... 

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