Two-phase fluidized bed reactors

Simulation model for two-phase fluidized bed reactor with single input single output proportional (SISOP) feedback control Figure 4.25... 

Fluidised bed reactors have an inherent advantage with higher heat transfer coefficients which is important due to the large amounts of heat that must be removed from the FT reactors to control their temperature. Fluid bed (also called fluidized bed) reactors may be two-phase (gas-catalyst) or three-phase (gas with catalyst suspended in a hydrocarbon liquid slurry). The three phase reactor is also known as a slurry phase reactor. It has been calculated that the heat transfer coefficient for the cooling surfaces in a slurry phase reactor are five times higher than those for fixed bed reactors (16-18). TTie magnitude of the heat transfer coefficient for a two phase fluidized bed reactor is similar to that for a slurry phase reactor. 

Figure 10.6 A schematic representation of the two-phase fluidized bed reactor model (FBMR) (E = emulsion phase, B = bubble phase).

Figure 6.31 shows the a schematic representation of this two-phase fluidized-bed reactor with a simple proportional control. It should be noted that the proportional control is based on the exit temperature (the average between the dense-phase and the bubble-phase temperatures), which is the measured variable, and the steam flow to the feed heater is the manipulated variable. 

In connection with the engineering content of the book, a large number of reactors is analyzed two- and three-phase (slurry) agitated reactors (batch and continuous flow), two-and three-phase fixed beds (fixed beds, trickle beds, and packed bubble beds), three-phase (slurry) bubble columns, and two-phase fluidized beds. All these reactors are applicable to catalysis two-phase fixed and fluidized beds and agitated tank reactors concern adsorption and ion exchange as well. 

A number of studies have been reported on solid-liquid mass transfer in different kinds of contactors, and they have been periodically reviewed, for example Miller (1971), Nienow (1975), Wen and Fan (1975), Briens et al. (1993). Of these, only the mechanically agitated contactor is used with or without a gas phase. It is the only truly two-phase solid-liquid contactor. The other types of contactors, such as the bubble-column contactor (usually), the trickle-bed reactor, and the three-phase fluidized-bed reactor, all involve three phases and are considered in Chapter 17 on multiphase reactions. 

Grace JR. Modelling and simulation of two-phase fluidized bed reaetors. In de Lasa HI, ed. Chemical Reactor Design and Technology. Dordrecht Martinus Nijhoff, 1986c, pp 245-289. 

In WAO with solid catalysts, three-phase reactors are used trickle bed, bubble slurry column, and bubble fixed-bed (monolith) or three-phase fluidized-bed reactors. When the catalyst is present in the liquid phase (homogeneous) or absent, two-phase reaetors such as bubble columns, jet-agitated reactors, and mechanically stirred reactor vessels are used. The limitations and advantages of these reactors for the application to WAO are listed in Table 10.7. 

The Union Carbide U.S. Patent 4,003,712 on a gas-phase fluidized-bed reactor was issued to Adam R. Miller on January 18, 1977. This patent was a continuation-in-part of two earlier patent applications one filed on August 21, 1967 and another filed on July 29, 1970, both of which were abandoned [36]. This design is shown in Figure 5.21. 

Gas-phase reactors for olefin polymerization are divided into two classes fluidized-bed reactors and stirred-bed reactors. The stirred-bed reactors can be further classified into vertical and horizontal. 

The first stage of the Spheripol process consists of polymerization in liquid propylene. Usually, two loops are used in series to narrow the residence-time distribution of the catalyst particles. For the ethylene-propylene copolymer (EPR) stage, the Spheripol process (Fig. 2.33) utilizes a gas phase fluidized bed reactor (FBR). The liquid propylene/ polymer suspension from the first reactor is flashed to gas/solid conditions prior to entering the second stage. The second stage operates at pressures of 15-35 atm, which is often close to the dew point of the gas. Elevated temperatures of approximately 80°C are used to provide a reasonable amount of copolymer contents in the final product. 

Sales et al. (2005) modeled the gas phase dynamic behavior of a three-phase fluidized bed reactor to evaluate the behavior of a gas tracer (methane) in the reactor. A two-zone model was proposed for the gas phase based on the operational behavior of the system. The dynamic model assumed that the total gaseous volume inside the reactor was distributed in two zones, represented by a tubular reactor of volume connected in series with a CSTR of volume The volumes of the two zones are related as... 

Equation 6-108 is also a good approximation for a fluidized bed reactor up to the minimum fluidizing condition. However, beyond this range, fluid dynamic factors are more complex than for the packed bed reactor. Among the parameters that influence the AP in a fluidized bed reactor are the different types of two-phase flow, smooth fluidization, slugging or channeling, the particle size distribution, and the... 

The copper was efficiently recovered more than 95-98% from the wastewater of electronic industries within 2-3 hrs as a powder by employing the three-phase inverse fluidized-bed reactors. The addition of a small amount of gas(Uc= 0.001) or fluidized particles(W=1.0wt.%) into the inverse fluidized bed reactors resulted in the increase of the copper recovery and decrease in the size of copper powda-recovered. The value of copper recovery exhibited a maximum value with increasing gas or liquid velocity, amount of fluidized particles or distance between the two electrodes, but it increased gradually with increasing current density up to 3.5A/dm between the two electrodes. The optimum conditions for the maximum recovery of copper powder were UG=0.001m/s, Ul= O.OOlm/s, W=1.0wt.%, I=3.5A/dm and LAc=0.015m within this experimental conditions. 

On the basis of different assumptions about the nature of the fluid and solid flow within each phase and between phases as well as about the extent of mixing within each phase, it is possible to develop many different mathematical models of the two phase type. Pyle (119), Rowe (120), and Grace (121) have critically reviewed models of these types. Treatment of these models is clearly beyond the scope of this text. In many cases insufficient data exist to provide critical tests of model validity. This situation is especially true of large scale reactors that are the systems of greatest interest from industry s point of view. The student should understand, however, that there is an ongoing effort to develop mathematical models of fluidized bed reactors that will be useful for design purposes. Our current... 

Fluidized beds are widely used to achieve either chemical reactions or physical processing that require interfacial contact between gas and particles. Heat transfer is important in many of these applications, either to obtain energy transfer between the solid and gas phases or to obtain energy transfer between the two-phase mixture and a heating/cooling medium. The latter case is particularly important for fluidized bed reactors which require heat addition or extraction in order to achieve thermal control with heats of reaction. 

In a fluidized bed reactor, entrained particles leaving in a dilute phase stream are conventionally and desirably either partially or wholly condensed into a bulk stream and returned to the bed via a centrifugally driven cyclone system. At equilibrium, or when steady state operation is attained, any particle loss rate from the cyclones, as well as the remaining bed particle size distribution, are functions of (a) the rate of any particle attrition within the system and (b) the smallest particle size that the cyclone system was designed to completely collect (i.e., with 100% efficiency), or conversely the largest size which the system cannot recover. These two functions result in an interdependency between loss rate and bed particle size distribution, eventually leading to an equilibrium state (Zenz Smith, 1972 Zenz, 1981 Zenz Kelleher, 1980). 

Obviously, these two items are not strictly separated in contrast, the most fruitful approach is when they are simultaneously followed, so that they can mutually benefit from each other. In this chapter, we want to focus on the use of simulation methods as a design tool for gas-fluidized bed reactors, for which we consider gas-solid flows at four distinctive levels of modeling. However, before discussing the multilevel scheme, it is useful to first briefly consider the numerical modeling of the gas and solid phase separately. 

An advantage of this approach to model large-scale fluidized bed reactors is that the behavior of bubbles in fluidized beds can be readily incorporated in the force balance of the bubbles. In this respect, one can think of the rise velocity, and the tendency of rising bubbles to be drawn towards the center of the bed, from the mutual interaction of bubbles and from wall effects (Kobayashi et al., 2000). In Fig. 34, two preliminary calculations are shown for an industrial-scale gas-phase polymerization reactor, using the discrete bubble model. The geometry of the fluidized bed was 1.0 x 3.0 x 1.0 m (w x h x d). The emulsion phase has a density of 400kg/m3, and the apparent viscosity was set to 1.0 Pa s. The density of the bubble phase was 25 g/m3. The bubbles were injected via 49 nozzles positioned equally distributed in a square in the middle of the column. 

Sadaka, S. S., Ghaly, A. E., and Sabbah, M. A., Two Phase Biomass Air-Steam Gasification Model for Fluidized Bed Reactors Part I - Model Development Biomass and Bioenergy, 22, 2002, pp. 439-462. 

TWO-PHASE, FLUID-SOLID FLUIDIZED BED REACTORS 3.8.1 General... 

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