Fluidized suspension reactors

Fluidized-bed catalytic reactors. In fluidized-bed reactors, solid material in the form of fine particles is held in suspension by the upward flow of the reacting fluid. The effect of the rapid motion of the particles is good heat transfer and temperature uniformity. This prevents the formation of the hot spots that can occur with fixed-bed reactors. 

For the discrete bubble model described in Section V.C, future work will be focused on implementation of closure equations in the force balance, like empirical relations for bubble-rise velocities and the interaction between bubbles. Clearly, a more refined model for the bubble-bubble interaction, including coalescence and breakup, is required along with a more realistic description of the rheology of fluidized suspensions. Finally, the adapted model should be augmented with a thermal energy balance, and associated closures for the thermophysical properties, to study heat transport in large-scale fluidized beds, such as FCC-regenerators and PE and PP gas-phase polymerization reactors. 

Chou and Wollast (1984) used a fluidized bed reactor to study albite weathering. An illustration of their device is shown in Fig. 3.5. The flow needed to maintain the feldspar particles in suspension is provided by the pumping rate Plt while P2 is the rate of addition of fresh solution P2 is also the rate of output of the reacted solution. By changing the rate of renewal of P2 one can vary the residence time of the fluid in the reactor. To maintain a small difference in concentration between the input at the bottom of the fluidized bed and the output at the top of the bed, P2 must be small in comparison to Pi. Chou and Wollast (1984) maintained the renewal rate P2 between 3 and 6% of the mixing rate Pi. 

Figure 3.5. Schematic representation of the fluidized bed reactor. Px is the rate required to keep the particles in suspension. P2 is the rate of addition of fresh input solution. [From Chou and Wollast (1984), with permission.]...

The principal reactors used are fluidized bed reactors, called Synthol reactors, in which the feed gas entrains an iron catalyst powder in a circulating flow. The suspension enters the bottom of the fluidized bed reaction section, where the Fischer-Tropsch and the gas shift reactions proceed at a temperature of from 315 to 330°C. These reactions are highly exothermic, as described previously, and the large quantity of heat released must be removed. The products in gaseous form together with the catalyst are taken off from the top of the reactor. By decreasing the gas velocity in another section, the catalyst settles out and is returned for reuse. The product gases are then condensed to the liquid products. 

From a practical point of view, a prolonged operation of a reactor with enzyme in suspension is not feasible. Procedures to retain the enzyme in the tubular reactor have been developed in order to maintain high enzymatic activity and to avoid enzyme washout. A plug-flow reactor operated with immobilized enzyme is known either as a fixed bed reactor or a fluidized bed reactor, depending on the characteristics of the flow pattern and the immobilized enzyme. Since mechanical stirring is not required in plug flow reactors, the support material is not damaged by the impeller, which may be a drawback in CSTR with immobilized enzyme. 

In a fluidized bed reactor, the immobilized enzyme is maintained in suspension by the inlet flow. Depending on the geometric characteristics of the reactor and the... 

In suspension reactors as well as in fluidized bed reactors for the free-falling velocity of a single particle has to be used. The free falling velocity of spheres can be found according to the following derivation. In the steady state the weight of the particle in the fluid is equal to the resistance to flow ... 

Photocatalytic reactions are promoted by solid photocatalyst particles that usually constitute the discrete phase distributed within a continuous fluid phase in the reactor. Therefore, at least two phases, that is, liquid and solid, are present in the reactor. The solid phase could be dispersed (SPD) or stationary (SPS) within the reactor. SPD photoreactors may be operated with the catalyst particles and the fluid phase(s) agitated by mechanical or other means. Depending on the means of agitation, the photoreactor resembles that of slurry or fluidized bed reactors. In numerous investigations, an aqueous suspension of the catalyst particles in immersion or annular-type photo reactors has been used. However, the use of suspensions requires the... 

The essential feature of a fluidized-bed reactor is that the solids are held in suspension by the upward flow of the reacting fluid this promotes high mass and heat transfer rates and good mixing. Heat transfer coeflicients in the order of 200 W/m-°C between jackets and internal coils are typically obtained. The solids may be a catalyst, a reactant (in some fluidized combustion processes), or an inert powder added to promote heat transfer. 

A hydrocracker can also be run with both liquid and gas fed from the bottom and the solids kept in suspension by drag forces of the fluids. This reactor is a fluidized bed reactor explained below. 

In this chapter the characteristics of fluidized gas-solid suspensions are described, and the basic designs of fluidized bed reactors are sketched. Several modeling approaches that have been applied to described these units are outlined. 

The three-phase fluidized-bed reactor (ebulliated-bed reactor) differs from suspension reactors in the use of larger catalyst particles (0.1 to 5 mm) and the formation of a well-defined agitated catalyst bed. Whereas suspension reactors can operate in both batch and continuous mode with regard to the liquid phase (and catalyst), the ebulliated-bed reactor only operates in continuous mode, and hence is generally not the appropriate choice for tire production of fine chemicals. 

As an alternative to the suspension process. Witco GmbH developed (1995) a technique which immobilizes the active compounds on a spray-dried silica support by utilizing a fluidized bed reactor. They claimed to produce supported metallocene catalysts with a controllable distribution of active centers achieved by using the three different supporting methods. Controlling the addition of tri-methylaluminum and water in the different reactor... 

Polymerization reactors are a specific kind of chemical reactors in which polymerization reactions take place therefore, in principle, they can be analyzed following the same general rules applicable to any other chemical reactor. The basic components of a mathematical model for a chemical reactor are a reactor model and rate expressions for the chemical species that participate in the reactions. If the system is homogeneous (only one phase), these two basic components are pretty much what is needed on the other hand, for heterogeneous systems formed by several phases (emulsion or suspension polymerizations, systems with gaseous monomers, slurry reactors or fluidized bed reactors with solid catalysts, etc.), additional transport and/or thermodynamic models may be necessary to build a realistic mathematical representation of the system. In this section, to illustrate the basic principles and components needed, we restrict ourselves to the simplest case, that of homogeneous reactors in other sections, additional components and more complex cases are discussed. 

In addition to suspension, a gas-phase process was developed. No diluent is used in the polymerization step. Highly purified ethylene gas is combined continuously with a dry-powdery catalyst and then fed into a vertical fluidized-bed reactor. The reaction is carried out at 270 psi and 85-100 °C. The circulating ethylene gas fluidizes the bed of growing granular polymer and serves... 

In Figure 2.25 the effectiveness factor as function of the Thiele modulus for different pellet shapes is shown. For small values of the Thiele modulus the effectiveness factor reaches unity in all cases. The reaction rate is controlled by the intrinsic kinetics, and the reactant concentration within the pellet is identical to the concentration at the outer pellet surface. This situation may be observed for low catalyst activity or very small particles as used in fluidized beds or suspension reactors. For large values of the Thiele modulus the dependency of r p approaches an asymptotic solution tjp = micp with w = 1, 2, 3 for a slab, a cylinder, and a sphere, respectively. This situation may occur for very fast reactions or large catalyst particles. The concentration in the center of the catalyst particles approaches zero for rip < 0.2. 

The so-called fluidized bed membrane reactor (FBMR) is obtained by immersing bundles of hydrogen-selective membranes into a gas—solid fluidized suspension (see Figure 3.7) (Gallucci et al., 2008a, 2008b). 

In a three-phase fluidized-bed reactor, also called an ebullated bed reactor, the solid catalyst particles are kept in suspension by the gas and the liquid flowing upwards (Figure 4.10.12b). The particles are relatively large (1-5 mm) to keep them from being carried away. Fluidized beds are preferred for strongly exothermic reactions or if the catalyst must be frequently exchanged because of rapid deactivation. 

In slurry reactors and in fluidized bed reactors, particles under a certain size may give rise to filtration problems. When there are severe heat transfer requirements, one may prefer a fine catalyst in a suspension that is well agitated a stirred slurry reactor, a bubble column or a fluidized bed. 

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