Catalytic reactors examples

For the heterogeneous system, the problem is very simple. We just write the above equation for each phase, taking into account Q as the heat transfer between the two phases. For a nonadiabatic system, the heat added from outside, Sexternai (which is the Q in Eq. (6.44)], will be added to the phase receiving it, or distributed between the two phases if it is added to both phases (this will depend very much on the configuration and knowledge of the physical system as will be shown with the nonadiabatic bubbling fluidized-bed catalytic reactor example). 

The second type of solution polymerization concept uses mixtures of supercritical ethylene and molten PE as the medium for ethylene polymerization. Some reactors previously used for free-radical ethylene polymerization in supercritical ethylene at high pressure (see Olefin POLYMERS,LOW DENSITY polyethylene) were converted for the catalytic synthesis of LLDPE. Both stirred and tubular autoclaves operating at 30—200 MPa (4,500—30,000 psig) and 170—350°C can also be used for this purpose. Residence times in these reactors are short, from 1 to 5 minutes. Three types of catalysts are used in these processes. The first type includes pseudo-homogeneous Ziegler catalysts. In this case, all catalyst components are introduced into a reactor as hquids or solutions but form soHd catalysts when combined in the reactor. Examples of such catalysts include titanium tetrachloride as well as its mixtures with vanadium oxytrichloride and a trialkyl aluminum compound (53,54). The second type of catalysts are soHd Ziegler catalysts (55). Both of these catalysts produce compositionaHy nonuniform LLDPE resins. Exxon Chemical Company uses a third type of catalysts, metallocene catalysts, in a similar solution process to produce uniformly branched ethylene copolymers with 1-butene and 1-hexene called Exact resins (56). 

As an example the use of ceramic membranes for ethane dehydrogenation has been discussed (91). The constmction of a commercial reactor, however, is difficult, and a sweep gas is requited to shift the product composition away from equiUbrium values. The achievable conversion also depends on the permeabihty of the membrane. Figure 7 shows the equiUbrium conversion and the conversion that can be obtained from a membrane reactor by selectively removing 80% of the hydrogen produced. Another way to use membranes is only for separation and not for reaction. In this method, a conventional, multiple, fixed-bed catalytic reactor is used for the dehydrogenation. After each bed, the hydrogen is partially separated using membranes to shift the equihbrium. Since separation is independent of reaction, reaction temperature can be optimized for superior performance. Both concepts have been proven in bench-scale units, but are yet to be demonstrated in commercial reactors. 

The various reaction rate properties of the different solvents influence the design of a catalytic reactor. Eor example, for a specific catalyst bed design, an effluent stream containing a preponderance of monohydric alcohols, aromatic hydrocarbons, or propjiene requires a lower catalyst operating temperature than that required for solvents such as isophorone and short-chain acetates. 

After the rates have been determined at a series of reactant concentrations, the differential method of testing rate equations is applied. Smith [3] and Carberry [4] have adequately reviewed the designs of heterogeneous catalytic reactors. The following examples review design problems in a plug flow reactor with a homogeneous phase. 

For the organic contaminants, the required bromine product quality wilt also be site specific. If the catalytic oxidation unit is dedicated to a single bromination process, phase separation and drying may be the only purification required. Contaminants in the recovered bromine which are either the starting materials or products of the original bromination reaction should not present a problem if present in bromine recycled to the bromination reactor. In this case, the catalytic reactor would be operated to minimize the formation of undesirable brominated byproducts. For example, if phenol is present in the waste HBr from a tribromo-phenol manufacturing process, minor tribromophenol contamination of the bromine recycled to the reactor should not be a problem. Similarly, fluorobenzene in bromine recycled to a fluorobenzene bromination process should not present a problem. 

Some examples for NMR techniques that have been used for investigating catalytic reactors (most of which will be discussed in this chapter) and the corresponding length scales which they probe are as follows ... 

There are circumstances when a complex process may involve two competing (i.e., opposing) dynamic effects that have different time constants. One example is the increase in inlet temperature to a tubular catalytic reactor with exothermic kinetics. The initial effect is that the exit temperature will momentarily decrease as increased conversion near the entrance region depletes reactants at the distal, exit end. Given time, however, higher reaction rates lead to a higher exit temperature. 

For gas phase heterogeneous catalytic reactions, the continuous-flow integral catalytic reactors with packed catalyst bed have been exclusively used [61-91]. Continuous or short pulsed-radiation (milliseconds) was applied in catalytic studies (see Sect. 10.3.2). To avoid the creation of temperature gradients in the catalyst bed, a single-mode radiation system can be recommended. A typical example of the most advanced laboratory-scale microwave, continuous single-mode catalytic reactor has been described by Roussy et al. [79] and is shown in Figs. 10.4 and... 

Fig. 6. Examples of types of meshes developed to resolve laminar flow around particles (a) Chimera grid. Reprinted, with permission, from the Annual Review of Fluid Mechanics, Volume 31 1999 by Annual Reviews www.annualreviews.org (b) Unstructured grid with layers of prismatic cells on particle surfaces. Reprinted from Chemical Engineering Science, Vol. 56, Calis et al., CFD Modeling and Experimental Validation of Pressure Drop and Flow Profile in a Novel Structured Catalytic Reactor Packing, pp. 1713-1720, Copyright (2001), with permission from Elsevier.

Examples of reactions carried out in fixed-bed catalytic reactors of various types are as follows ... 

It is not practical to stir all reaction systems, for example, bulk polymerizations, postpolymerization reactions, fixed-bed catalytic reactors, and plug-flow reactors. Although multipoint temperature sensing is often used as a key solution to determine a runaway in nonagitated vessels, the occurrence of hot spots may not always be detected. 

The structure of this review is as follows. Section II focuses on the basic principles of MRI techniques, and then more advanced techniques which have been used to study catalytic reactors will be introduced in Section III. To illustrate the use of these techniques examples will be given from the field of catalysis, although not necessarily at the scale of the reactor and, in some cases, data for model systems will be shown. Section IV describes methods used to achieve chemical mapping. Section V focuses exclusively on previous research which has used MRI techniques to spatially resolve chemical composition in fixed-bed reactors. 

The concentration and temperature Tg will, for example, be conditions of reactant concentration and temperature in the bulk gas at some point within a catalytic reactor. Because both c g and Tg will vary with position in a reactor in which there is significant conversion, eqns. (1) and (15) have to be coupled with equations describing the reactor environment (see Sect. 6) for the purpose of commerical reactor design. Because of the nonlinearity of the equations, the problem can only be solved in this form by numerical techniques [5, 6]. However, an approximation may be made which gives an asymptotically exact solution [7] or, alternatively, the exponential function of temperature may be expanded to give equations which can be solved analytically [8, 9]. A convenient solution to the problem may be presented in the form of families of curves for the effectiveness factor as a function of the Thiele modulus. Figure 3 shows these curves for the case of a first-order irreversible reaction occurring in spherical catalyst particles. Two additional independent dimensionless paramters are introduced into the problem and these are defined as... 

Figure 12.10 Examples of structured catalytic reactors for kinetic measurements (a) annular reactor [47, 61] (b) plate cell reactor [75].

Mass transfer steps are essential in any multiphase reactor because reactants must be transferred from one phase to another. When we consider other multiphase reactors in later chapters, we will see that mass transfer rates fiequently control these processes. In this chapter we consider a simpler example in the catalytic reactor. This is the first example of a multiphase reactor because the reactor contains both a fluid phase and a catalyst phase. However, this reactor is a very simple multiphase reactor because the catalyst does not enter or leave the reactor, and reaction occurs only by the fluid reacting at the catalyst surface. 

The catalytic reactor is an example where reaction occurs only at the boundary with a solid phase, but, as long as the solid remains in the reactor and does not change, we did not need to write separate mass balances for the soHd phase because its residence time Tj is infinite. In a moving bed catalytic reactor or in a slurry or fluidized bed catalytic reactor... 

The most common unit of <2rel is bed volumes per hour (BV/li). Space velocity is also used in catalytic reactors, especially in three-phase fixed-beds, and is referred to as liquid hourly space velocity (LHS V) for the liquid phase, and gas hourly space velocity (GHSV) for gas phase. As mentioned above, space-time and space velocity are measured under the entrance conditions. However, for space velocity, other conditions are frequently used (Fogler, 1999). For example, the LHSV is measured at 60 to 75 °F, and GHSV at standard temperature and pressure. 

Traditionally the technique of the medical physicist, magnetic resonance imaging (MRI) has long been used to investigate the internal structure of the human body and the transport processes occurring within it for example, MRI has been used to characterize drug transport within damaged tissue and blood flow within the circulatory system. It is therefore a natural extension of medical MRI to implement these techniques to study flow phenomena and chemical transformations within catalysts and catalytic reactors. 

Tortuosity factors less than unity have often been reported for three-phase catalytic reactors where a reactant gas diffuses in a liquid- (or dense) medium. Many examples are available. A... 

In the third section an extensive writing on two types of slurry catalytic reactors is proposed Bubble Slurry Column Reactors (BSCR) and Mechanically Stirred Slurry Reactors (MSSR). All the variables relevant in the design and for the scale-up and the scale-down of slurry catalytic reactors are discussed particularly from the point of view of hydrodynamics and mass transfer. Two examples of application are included at the end of the section. 

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