Pneumatic transport reactor

Figure 23.2 depicts some of the essential features of (a) fluidized-bed, (b) fast-fluidized-bed, and (c) pneumatic-transport reactors (after Yates, 1983, p. 35). 

Figure 23.2 Some features of (a) a fluidized-bed reactor (b) a fast-fluidized-bed reactor and (c) a pneumatic-transport reactor...

Bermti et al [13], for example, used the term CFB to generically describe systems like fast fluidized bed, riser reactor, entrained bed, transport bed, pneumatic transport reactor, recirculating solid riser, highly expanded fluid bed, dilute phase transported bed, transport line reactor and suspended catalyst bed in co-current gas flow. 

Koyama, H. and Dranoff, J. S., Modeling the Thermal Cracking of Ethane and Propane in a Non-Isothermal Vertical Pneumatic Transport Reactor , Ind. Eng. Chem. Res., 31, 2,265(1992). 

Multi-environment systems with two flowing phases. These systems are perhaps of most interest in reaction engineering applications since they include the most frequently used multiphase reactors. Gas-liquid bubble columns, ebullated beds, three-phase fluidized beds, gas-lift slurry reactors, trickle-bed reactors, pneumatic transport reactors, etc. fall into this category. Some of the developments presented in Section 6.1.1 can be extended to treat these systems. The multivariable joint p.d.f. has to be defined taking into the account that the system has multiple inlets and outlets, i.e. by following the rules established in Section 3 by the appropriate extension of eqs. (9) and (10). However, this approach has not been presented or used to date. The main reason is that the transforms do not have a readily useable analytical form and are functions of many system... 

When a chemical reaction occurs in the system, each of these types of behavior gives rise to a corresponding type of reactor. These range from a fixed-bed reactor (Chapter 21-not a moving-particle reactor), to a fluidized-bed reactor without significant carryover of solid particles, to a fast-fluidized-bed reactor with significant carryover of particles, and ultimately a pneumatic-transport or transport-riser reactor in which solid particles are completely entrained in the rising fluid. The reactors are usually operated commercially with continuous flow of both fluid and solid phases. Kunii and Levenspiel (1991, Chapter 2) illustrate many industrial applications of fluidized beds. 

Viscosity. Dense phase solid-gas mixtures may be required to flow in transfer line catalytic crackers, between reactors and regenerators and to circulate in dryers such as Figures 9.13(e), (f). In dilute phase pneumatic transport the effective viscosity is nearly that of the fluid, but that of dense phase mixtures is very much... 

The physical transport of the reactor products may also need sufficient processing. Bulk polymerizations in high-pressure extruders may be followed by dicing the product into small pellets suitable for pneumatic transport. Emulsion and solution products are usually transported as obtained and frequently used in the same mode. The design of suitable containers and transportation protocols is very important to avoid harming the product during transportation and storage. For example, special containers may be required for air-sensitive materials, or to avoid solvent or water loss from solutions and emulsions, respectively. 

In vertical pneumatic transport the radial particle concentration distribution is almost uniform, but some particle strands may still be identified near the wall. Little or no axial variation of solids concentration except in the bottom acceleration section is observed [58]. The flow associated with transport bed reactors tends to be dilute (typically 1 to 5 % by volume solids) and uniform. By virtue of the smaller reflux and density of the suspension within the dilute pneumatic conveying regime, there might be larger temperature gradients than within the fast fluidization regime [56]. 

For pneumatic conveying all the particles are evenly dispersed in the gas. This makes contacting ideal or close to ideal. The plug flow model is thus well suited for the dilute transport reactors, but has also been used for the denser fast fluidization regime neglecting gradients in the solids distribution. For first order reactions the model can be written as ... 

In the entrained-flow reactor (Figure 18.11), the solid particles travel with the reacting fluid through the reactor. Such a reactor has also been described as a dilute or lean-phase fluidized bed with pneumatic transport of solids. 

Figure 7 Nuclear reactor Thetis dedicated to neutron activation analysis. View in the swimming pool with nuclear fuel element, Cerenkov radiation, safety and control plates and pneumatic transport tubes. (Reproduced with permission from Institute of Nuclear Sciences, Gent, Belgium.)...

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