Heterogeneous tubular reactor

If the reaetion in a tubular reaetor proeeeds in a solid eatalyst (e.g., in the form of a paeked bed of eatalyst pellets), the eonversion rate is often given per unit mass of solids (-1a). In this ease, the total mass of solids Ml required for a eertain degree of eonversion is 

The reactor volume is calculated from Mj and the bulk density of the catalyst material, (-r ) depends not only on composition and temperature, but also on the nature and size of the catalyst pellets and the flow velocity of the mixture. In a heterogeneous reaction where a solid catalyst is used, the reactor load is often determined by the term space velocity, SV. This is defined as the volumetric flow at the inlet of the reactor divided by the reaction volume (or the total mass of catalyst), that is 

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Adiabatic plug flow reactors operate under the condition that there is no heat input to the reactor (i.e., Q = 0). The heat released in the reaction is retained in the reaction mixture so that the temperature rise along the reactor parallels the extent of the conversion. Adiabatic operation is important in heterogeneous tubular reactors. 

A heterogeneous tubular reactor that incorporates three phases where gas and liquid reactants are contacted with the solid catalyst particles, is classified as a trickle-bed reactor. The liquid is usually allowed to flow down over the bed of catalyst, while the gas flows either up or down through the void spaces between the wetted pellets. Co-current downflow of the gas is generally preferred because it allows for better distribution of liquid over the catalyst bed and higher liquid flow rates are possible without flooding. 

This comparison focuses on the comer regions in square ducts that are nonexistent in tubes. In both configurations, the momentum boundary layer thickness is substantial (i.e., Effective/2) for fully developed laminar flow. The no-slip boundary condition for viscous flow near the walls increases the mass transfer boundary layer thickness and reduces the flux of reactants toward the catalytic surface relative to plug flow. This effect is significant in the comer regions of the channel with square cross section. Since the entire active surface in heterogeneous tubular reactors is equally accessible to reactants, one predicts larger conversion in tubes via equation (23-71) ... 

Various experimental methods to evaluate the kinetics of flow processes existed even in the last centuty. They developed gradually with the expansion of the petrochemical industry. In the 1940s, conversion versus residence time measurement in tubular reactors was the basic tool for rate evaluations. In the 1950s, differential reactor experiments became popular. Only in the 1960s did the use of Continuous-flow Stirred Tank Reactors (CSTRs) start to spread for kinetic studies. A large variety of CSTRs was used to study heterogeneous (contact) catalytic reactions. These included spinning basket CSTRs as well as many kinds of fixed bed reactors with external or internal recycle pumps (Jankowski 1978, Berty 1984.)... 

Chapter 1 reviews the concepts necessary for treating the problems associated with the design of industrial reactions. These include the essentials of kinetics, thermodynamics, and basic mass, heat and momentum transfer. Ideal reactor types are treated in Chapter 2 and the most important of these are the batch reactor, the tubular reactor and the continuous stirred tank. Reactor stability is considered. Chapter 3 describes the effect of complex homogeneous kinetics on reactor performance. The special case of gas—solid reactions is discussed in Chapter 4 and Chapter 5 deals with other heterogeneous systems namely those involving gas—liquid, liquid—solid and liquid—liquid interfaces. Finally, Chapter 6 considers how real reactors may differ from the ideal reactors considered in earlier chapters. 

Denis, G. H. Kabel, R. L. 1970 The effect of temperature changes on a tubular heterogeneous catalytic reactor. Chem. Engng Sci. 25, 1057-1071. 

Figure 4-13. Liquid-liquid heterogeneous tubular flow reactor (e.g., alkylation of olefins and isobutane). (Source J. M. Smith, Chemical Engineering Kinetics, 3rd ed., McGraw-Hill, Inc., 1981.)...

Example 44 Dimensioning of a tubular reactor, equipped with a mixing nozzle, designed for carrying out competitive-consecutive reactions 193 Example 45 Mass transfer limitation of the reaction rate of fast chemical reactions in the heterogeneous material system gas/liquid 197... 

As shown in Table 3.1, tubular reactors could be of different types. A very large number of heterogeneous catalytic reactions are carried out in a tubular reactor of one type or another. In tubular reactors the reactant is fed in from one end of the tube with the help of a pump, and the product is removed from the other end of the tube. The other reactants, if there are any, may be introduced either at the reactor entrance in a cocurrent fashion, or from the reactor exit in a countercurrent fashion (Fig. 3.3). 

Chemical vapor deposition is a key process for thin film formation in the development and manufacture of microelectronic devices. It shares many kinetic and transport phenomena with heterogeneous catalysis, but CVD reactor design has not yet reached the level of sophistication used in analyzing heterogeneous catalytic reactors. With the exception of the tubular LPCVD reactor, conventional CVD reactors may be viewed as variations on the original horizontal reactor. These reactors have complex flow fields and it is consequently difficult to control and predict the effect of operating conditions on the film thickness and composition. 

It must be emphasized that while immobilized enzymes have advantages, they are not always economical in practice. An example given by Gould and Rocks (12) is worth repeating here to illustrate this point. The use of an open tubular heterogeneous enzyme reactor 2is a component of an autoanalyzer... 

Jensen and Ray (50) have recently tabulated some 25 experimental studies which have demonstrated steady state multiplicity and instabilities in fixed-bed reactors many of these (cf., 29, 51, 52) have noted the importance of using a heterogeneous model in matching experimental results with theoretical predictions. Using a pseudohomogeneous model, Jensen and Ray (50) also present a detailed classification of steady state and dynamic behavior (including bifurcation to periodic solutions) that is possible in tubular reactors. 

The principal use of tubular reactors for kinetic studies is as catalytic fixed-bed reactors in heterogeneous catalysis. They are rarely used for quantitative studies of homogeneous reactions because these are difficult to confine sharply to reactors of this type (see farther below). 

Differential reactors are primarily used for studies of heterogeneous catalysis, Homogeneous reactions are very difficult to confine as sharply as necessary to a very small flow reactor. An exception are radiation-induced reactions (see comment in preceding section on tubular reactors). 

It should be noted that Equations 1, 2, 3, 4, and 5 imply a homogeneous kinetic system. Coking in tubular reactors results from a combination of homogeneous and heterogeneous processes. As the kinetics of these processes are not well understood and as the quantitative yield of coke is several orders of magnitude smaller than other pyrolysis products, it is more convenient to model coke formation separately based on commercial operating data. 

A tubular reactor packed with a heterogeneous catalyst is accomplishing an oxidation reaction of an alcohol to an aldehyde. The reactions are ... 

I Homogeneous versus Heterogeneous Reactions in Tubular Reactors... 

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