Other Tubular Reactor Configurations

It is sometimes possible to overcome the problem of low Xc with adiabatic operation by using several adiabatic reactors with cooling between reactors. Thus one runs the first reactor 

There are several examples of exothermic reversible reactors where interstage cooling is practiced. One is methanol synthesis from syngas 

As discussed in Chapter 3, this reaction is reversible at 250°C, where the commercial reactors are operated, and it is common practice to use interstage cooling and cold feed injection to attain high conversion and extract reaction heat. Another important reaction is the water-gas shift, used to prepare industrial H2 from syngas, 

This process is typically run with two reactors, one at 800°C, where the rate of the reaction is high, and the second at 250°C, where the equilibrium conversion is higher. 

We have discussed these reactions previously in connection with equilibrium limitations on reactions, and we will discuss them again in Chapter 7, because both use catalysts. These reactions are very important in petrochemicals, because they are used to prepare industrial H2 and CO as well as methanol, formaldehyde, and acetic acid. As noted previously, these processes can be written as 

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Other configurations that are used include an concentric electrode setup in a tubular reactor, where the discharge still is capacitivily coupled. Also, inductive coupling has been used, with a coil surrounding the tubular reactor [146, 147]. 

Other advantages of the tubular reactor relative to stirred tanks include suitability for use at higher pressures and temperatures, and the fact that severe energy transfer constraints may be readily surmounted using this configuration. The tubular reactor is usually employed for liquid phase reactions when relatively short residence times are needed to effect the desired chemical transformation. It is the reactor of choice for continuous gas phase operations. 

Exxon and Phillips manufacture polypropylene in tubular reactors where the monomer is in the liquid form (see Section 6.8.2). One of the manufacturing processes for polyethylene involves the use of a loop reactor that has a recycle configuration. Here, under elevated pressure and temperature, a mixture of the catalyst, comonomer, hydrogen, and a solvent are introduced from one end of the reactor. The product and the unreacted starting materials are collected at the other end, and recycled back into the reactor. 

Tubular reactors are also used to carry out some multiphase reactions. Wamecke et al. (1999) reported use of a computational flow model to simulate an industrial tubular reactor carrying out a gas-liquid reaction (propylene oxide manufacturing process). In this process, liquid is a dispersed phase and gas is a continuous phase. The two-fluid model discussed earlier may be used to carry out simulations of gas-liquid flow through a tubular reactor. Warnecke et al. (1999) applied such a model to evaluate the influence of bends etc. on flow distribution and reactor performance. The model may be used to evolve better reactor configurations. In many tubular reactors, static mixers are employed to enhance mixing and other transport processes. Computational flow models can also make significant contributions to understanding the role of static mixers and for their optimization. Visser et al. (1999) reported CFD... 

The analysis of chemical reactor operations is limited to simple reactor configurations (i.e., batch, tubular, CSTR), with little, if any analysis, of other configurations (i.e., semibatch, tubular with side injection, distillation reactor),... 

A systematic approach to the design of a reactor should start by discussing the field of velocity distributions. Much progress has been achieved in this area and the hydrodynamic characterization of a great variety of reactors is already known. For the sake of brevity, we will concentrate on two types of systems a perfectly mixed reaction space and a fully developed unidirectional flow in a tubular reactor. In practical terms this is not a serious limitation in computational fluid mechanics, commercially available calculating codes can be used to solve almost any other form of reactor configuration. 

A pilot plant scale, tubular (annular configuration) photoreactor for the direct photolysis of 2,4-D was modeled (Martin etal, 1997). A tubular germicidal lamp was placed at the reactor centerline. This reactor can be used to test, with a very different reactor geometry, the kinetic expression previously developed in the cylindrical, batch laboratory reactor irradiated from its bottom and to validate the annular reactor modeling for the 2,4-D photolysis. Note that the radiation distribution and consequently the field of reaction rates in one and the other system are very different. 

The Dowlex process by Dow Chemicals is the dominant process in solution polymerization, but Dow does not license this technology to other companies (Figure 2.39). The Dowlex process uses two CSTRs in series with a high boiling hydrocarbon solvent. Other competing processes include the DSM process and the Sclairtech process by Nova Chemicals. In some configurations, these processes may also have tubular reactors operated in series with the CSTR to complete monomer conversion. 

The polymerization of olefins with coordination catalysts is performed in a large variety of polymerization processes and reactor configurations that can be classified broadly into solution, gas-phase, or slurry processes. In solution processes, both the catalyst and the polymer are soluble in the reaction medium. These processes are used to produce most of the commercial EPDM rubbers and some polyethylene resins. Solution processes are performed in autoclave, tubular, and loop reactors. In slurry and gas-phase processes, the polymer is formed around heterogeneous catalyst particles in the way described by the multigrain model. Slurry processes can be subdivided into slurry-diluent and slurry-bulk. In slurry-diluent processes, an inert diluent is used to suspend the polymer particles while gaseous (ethylene and propylene) and liquid (higher a-olefins) monomers are fed into the reactor. On the other hand, only liquid monomer is used in the slurry-bulk pro-... 

Other variables of importance in designing these tubular pyrolysis reactors include the mass velocity (or flow velocity) of the gaseous reaction mixture in the tubes, pressure, steam-to-hydrocarbon-feedstock ratio, heat flux through the tube wall, and tube configuration and spacing. Pressure drop in the reactor is of major importance, especially because of the extremely high flow velocities normally employed. 

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