Nonisothermal reactors packed

A one-dimensional one-phase dispersion model subject to the Danckwerts boundary conditions has been used for a description of the dynamics of a nonisothermal nonadiabatic packed bed reactor. The dimensionless governing equations are ... 

Modeling of Tubular Nonisothermal Nonadiabatic Packed-Bed Reactors with Catalyst Poisoning... 

FIGURE 18.16 Distribution of reactants inside a packed-bed nonisothermal reactor during the electrochemical reduction of ethylene. 

To illustrate the above points in a simple manner, let us consider a nonisothermal adiabatic fixed-bed reactor packed with nonporous catalyst pellets for a simple exothermic reaction... 

A model that includes all the above terms is normally called the radial axial flow packed reactor (RAFPR) and represents the most sophisticated fixed-bed reactor model. The coupled differential equations for momentum balance, component balance, and for a nonisothermal reactor, the energy balance must be simultaneously solved (Li, 2007 Singh, 2005). 

NONISOTHERMAL HYDRATION OP NITROBENZENE IN A PACKED BED TUBULAR REACTOR... 

There may be radial temperature gradients in the reactor that arise from the interaction between the energy released by reaction, heat transfer through the walls of the tube, and convective transport of energy. This factor is the greatest potential source of disparities between the predictions of the model and what is observed for real systems. The deviations are most significant in nonisothermal packed bed reactors. 

Chen et al. [70] suggested that temperature gradients may have been responsible for the more than 90 % selectivity of the formation of acetylene from methane in a microwave heated activated carbon bed. The authors believed that the highly nonisothermal nature of the packed bed might allow reaction intermediates formed on the surface to desorb into a relatively cool gas stream where they are transformed via a different reaction pathway than in a conventional isothermal reactor. The results indicated that temperature gradients were approximately 20 K. The nonisothermal nature of this packed bed resulted in an apparent rate enhancement and altered the activation energy and pre-exponential factor [94]. Formation of hot spots was modeled by calculation and, in the case of solid materials, studied by several authors [105-108],... 

The main difficulties of design of catalytic reactors reduce to the following two questions (1) How do we account for the nonisothermal behavior of packed beds and (2) How do we account for the nonideal flow of gas in fluidized beds. 

For packed bed reactors, Carberry and Wendel (1963), Hlavacek and Marek (1966), and Carberry and Butt (1975) report that axial dispersion effects are negligible if the reactor length is sufficient. These and other researchers (Young and Finlayson, 1973 Mears, 1976) have developed criteria based on the reactor length for conditions where axial dispersion can safely be neglected. The criterion shown in Table V is a classic criterion for neglecting axial mass dispersion. The works by Young and Finlayson (1973) and Mears (1976) provide more detailed criteria to predict when axial dispersion is unimportant in nonisothermal packed bed reactors. 

It is the purpose of this chapter to discuss presently known methods for predicting the performance of nonisothermal continuous catalytic reactors, and to point out some of the problems that remain to be solved before a complete description of such reactors can be worked out. Most attention will be given to packed catalytic reactors of the heat-exchanger type, in which a major requirement is that enough heat be transferred to control the temperature within permissible limits. This choice is justified by the observation that adiabatic catalytic reactors can be treated almost as special cases of packed tubular reactors. There will be no discussion of reactors in which velocities are high enough to make kinetic energy important, or in which the flow pattern is determined critically by acceleration effects. 

If the flow rate is sufficiently high to create turbulent flow, then Pe is a constant and the magnitude of the right-hand side of the equation is determined by the aspect ratio, L/d. By solving Equation, (8.4.12) and comparing the results to the solutions of the PER [Equation (8.4.3)], it can be shown that for open tubes, L/d, > 20 is sufficient to produce PER behavior. Likewise, for packed beds, L/d, > 50 (isothermal) and L d, >150 (nonisothermal) are typically sufficient to provide PER characteristics. Thus, the effects of axial dispersion are minimized by turbulent flow in long reactors. 

The Hrst type of generic model for shell-and-tube membrane reactors refers to a nonisothermal packed-bed catalytic membrane tubular reactor (PBCMTR) whose cross-sectional view is shown in Figure lO.l. Mathematical models for this type of membrane reactor have been reviewed quite extensively by Tsotsis et al. [1993b]. 

Equation (11.56) shows the dimensionless equations for a nonisothermal packed bed reactor in a cylindrical domain ... 

The classic landmark paper on parametric sensitivity in nonisothermal chemical reactors is by Bilous and Amundson (1956). A more recent example of multiple stationary states in packed catalytic tubular reactors is discussed by Pedernera et al. (1997). 

DESIGN OF A NONISOTHERMAL PACKED CATALYTIC TUBULAR REACTOR 745... 

TABLE 27-9 System of Equations to Be Analyzed to Design a Packed Catalytic Tiibular Reactor That Operates Nonisothermally... 

Tubular reactors are normally used in the chemical industry for extremely large-scale processes. When filled with solid catalyst particles, such reactors are referred to as fixed-bed or packed-bed reactors. In this section we treat general design relationships for tubular reactors in which isothermal homogeneous reactions take place. Nonisothermal tubular reactors are treated in Section 10.4 and packed-bed reactors in Section 12.7. 

The simplest manifestation of the nonisothermal nature of a packed bed is the presence of temperature gradients at the reactor scales (axially over the bed length L and radially across its... 

Mears DE. On criteria for axial dispersion in nonisothermal packed-bed catalytic reactors. Industrial and Engineering Chemistry Fundamentals 1976 15 20-23. 

Young LC, Finlayson BA. Axial dispersion in nonisothermal packed bed chemical reactors. Industrial Engineering Chemistry Fundamentals 1973 12 412-422. 

The most important fixed-bed designs are the nonisothermal, nonadiabatic, fixed- (or packed)-bed reactor (NINA-PBR) (also called the multitubular or heat-exchanger-type reactor), and the single or multistage adiabatic fixed-bed reactor (A-PBR), and it is important at the outset to note the difference between the approaches and the design of these two operational categories. 

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