Fluidized catalyst beds properties

The design of reactors, preparation of catalysts, control of tempera-tim and other topics of practical importance are summarized by Pokrovskii in excellent reviews1 84.1885 which encompass the literature up to 13o5. Reference should be made to these sources for numerous patent disclosures that will not be considered in the present disoussicn Among the significant problems examined by Pokrovskii from the standpoint of industrial technology are relative merits of fixed and fluidized catalyst beds, optimum composition of the reaction mixture in terms of both yield and safety, and properties of catalysts—selectivity, activity, durability, etc,—that arc vita] to the success of the enterprise. 

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. 

In the preceding section, the flow properties of fluidized catalyst beds have been clarified mainly on the basis of experimental observations. In the case of FCC catalyst, the apparent viscosity of the emulsion is usually very small, and the emulsion shows good fluidity. Catalyst particles once charged into a fluidized bed reactor are usually in service for several months. Hence it is justifiable to prepare the particles very carefully, so that the fluidized bed shows the best fluidization possible. This kind of careful preparation is usually impractical in the case of single-pass particles such as coal, mineral ores, or grain. 

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. 

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-... 

A gas bubble column is taken here as a model equipment undergoing longitudinal dispersion of the continuous phase. The theory obtained is equally applicable to a fluidized catalyst bed of good fluidity exhibiting similar flow properties. The following procedure is from Miyauchi (M27). 

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. 

Scientific approaches to improve bed fluidity are potentially important for fluidized bed technology. Also, further quantitative relations between bubble splitting and bed properties would be very helpful in planning and scaling-up fluidized catalyst 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. 

The object of the following treatment is to establish a physically sound reactor model to obtain A or> based on the flow and transport properties of fluidized catalyst beds. Bed performance for chemical kinetics other than the first-order reaction may be computed after a sound bed performance has been established. 

A reactor model is developed to include reaction taking place in the dilute phase, and to be reasonably consistent with the known flow properties of fluidized catalyst beds operated under relatively high gas velocity. According to this model, reaction proceeds successively in the dense phase and in the dilute phase. 

The concept of the successive contact mechanism has been given its simplest form by dividing the fluidized catalyst bed into two parts—dense phase and dilute phase. The concept has been found to apply to bed performance, as shown in the preceding section. The reactor model has been developed on the basis of several simplifying assumptions, partly to retain mathematical simplicity as a workable design equation accounting for the relative effects of the variables, and partly due to a relative lack of information about bed performance. Further properties of the mechanism are examined here, particularly as to axial distribution of reactivity inside the bed. 

A gradual improvement in activity and selectivity was noted up to 36 hours, and slow continued improvement is quite probable beyond this point. The amount of neohexane and toluene in the liquid effluent continued to decrease with time, indicating some change in catalyst properties. During the test, temperature traverses were made throughout the fluidized catalyst bed. Temperatures from top to bottom of the bed varied by 2°F. 

Fluidized-bed catalytic cracking units (FCCUs) are the most common catalytic cracking units. In the fluidized-bed process, oil and oil vapor preheated to 500 to SOOT is contacted with hot catalyst at about 1,300°F either in the reactor itself or in the feed line (called the riser) to the reactor. The catalyst is in a fine, granular form which, when mixed with the vapor, has many of the properties of a fluid. The fluidized catalyst and the reacted hydrocarbon vapor separate mechanically in the reactor and any oil remaining on the catalyst is removed by steam stripping. 

Since the catalyst is so important to the cracking operation, its activity, selectivity, and other important properties should be measured. A variety of fixed or fluidized bed tests have been used, in which standard feedstocks are cracked over plant catalysts and the results compared with those for standard samples. Activity is expressed as conversion, yield of gasoline, or as relative activity. Selectivity is expressed in terms of carbon producing factor (CPF) and gas producing factor (GPF). These may be related to catalyst addition rates, surface area, and metals contamination from feedstocks. 

A fluidized bed reactor contains catalyst particles with a mean diameter of 500 pm and a density of 2.5 g/cm3. The reactor feed has properties equivalent to 35° API distillate at 400°F. Determine the range of superficial velocities over which the bed will be in a fluidized state. 

Attrition of particulate materials occurs wherever solids are handled and processed. In contrast to the term comminution, which describes the intentional particle degradation, the term attrition condenses all phenomena of unwanted particle degradation which may lead to a lot of different problems. The present chapter focuses on two particular process types where attrition is of special relevance, namely fluidized beds and pneumatic conveying lines. The problems caused by attrition can be divided into two broad categories. On the one hand, there is the generation of fines. In the case of fluidized bed catalytic reactors, this will lead to a loss of valuable catalyst material. Moreover, attrition may cause dust problems like explosion hazards or additional burden on the filtration systems. On the other hand, attrition causes changes in physical properties of the material such as particle size distribution or surface area. This can result in a reduction of product quality or in difficulties with operation of the plant. 

Hybrid catalysts consisting of a zeolite (ZSM-5 or Beta) and bentonite as a binder were prepared and characterized by XRD, pyridine FTIR and nitrogen adsorption. The hybrid catalysts exhibited similar properties as the combined starting materials. Catalytic pyrolysis over pure ZSM-5 and Beta as well as hybrid catalysts has been successfully carried out in a dual-fluidized bed reactor. De-oxygenation of the produced bio-oil over the different zeolitic materials was increased compared to non-catalytic pyrolysis over quartz sand. 

Several different reactor types were used for catalyst evaluation, including a DCR pilot riser [3] an ACE fixed fluidized bed (FFB) reactor [7], a Riser simulator [4,9], and a specially designed extended residence time circulating pilot unit. The reaction conditions of each of the reactors will be reported in the sections dealing with the specific reactor type. Different grades of Brazilian Campos Basin derived VGOs were used in the experiments. Feed properties are presented in Table 2.1. 

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