Slurry reactors rate-limiting step

The carrier can be in the form of a powder used for slurry reactions or a particulate such as a sphere, cylinder, or tablet (typically a few mm in diameter) used in fixed bed reactors. The size and shape depend very much on what is anticipated to be the rate-limiting step. For example, for a reaction limited by pore diffusion it is customary to use a smaller particle in the shape of a star, trilobe, or wagon wheel to decrease the diffusion path while increasing the external geometric surface area. Mechanical strength and solubility... 

Of primary interest for the industrial application of monolith reactors is to compare them with other conventional three-phase reactors. Two main categories of three-phase reactors are slurry reactors, in which the solid catalyst is suspended, and packed-bed reactors, where the solid catalyst is fixed. Generally, the overall rate of reactions is often limited by mass transfer steps. Hence, these steps are usually considered in the choice of reactor type. Furthermore, the heat transfer characteristics of chemical reactors are of essential importance, not only due to energy costs but also due to the control mode of the reactor. In addition, the ease of handling and maintenance of the reactor have a major role in the choice of the reactor type. More extensive treatment of conventional reactors can be found in the works by Gianetto and Silveston [11], Ramachandran and Chaudhari [12], Shah [13,14], Shah and Sharma [15], and Trambouze et al. [16], among others. 

A complete description of the slurry reactor and the transport and reaction steps are given on the CD-ROM. along with the design equations and a number of examples. Methods to determine which of the transpon and reaction steps are rate limiting are included. See Professional Reference Shelf R12.1. 

Closure After completing this chapter, the reader should be able to derive differential equations describing diffusion and reaction, discuss the meaning of the effectiveness factor and its relationship to the Thiele modulus, and identify the regions of mass transfer control and reaction rate control. The reader should be able to apply the Weisz-Prater and Mears criteria to identify gradients and diffusion limitations. These principles should be able to be applied to catalyst particles as well as biomaierial tissue engineering. The reader should be able to apply the overall effectiveness factor to a packed bed reactor to calculate the conversion at the exit of the reactor. The reader should be able to describe the reaction and transport steps in slurry reactors, trickle bed reactors, fluidized-besd reactors, and CVD boat reactors and to make calculations for each reactor. 

Mass (and heat) transport steps sometimes can be rate limiting in agitated and aerated slurry reactors.Therefore methods have to be provided to determine the relative importance of these steps and furthermore equations have to be at hand for the estimation of the special transport parameters in their dependence on physical properties, operating conditions and reactor geometry. 

In may be noted that a level [I] reactor selection can be done even with the effective reaction rate expressions (Equations 6.3 and 6.4). For instance, one should always attempt to select a reactor that helps to quicken the otherwise slowest step in the effective rate. For instance, if internal diffusion within catalyst particles is the limiting step, then one has to use fine particles in a slurry bubble column. If liquid-solid mass transfer is... 

Special consideration needs to be given to heterogeneous reactors, in which interaction of the phases is required for the reactions to proceed. In these situations, the rate of reaction may not be the deciding factor in the reactor design. The rate of transport of reactants and products from one phase to another can limit the rate at which products are obtained. For example, if reactants cannot get to the surface of a soHd catalyst faster than they would react at the surface, then the overall (observed) rate of the process is controlled by this mass transfer step. To improve the rate, the mass transfer must be increased. It would be useless to make changes that would affect only the surface reaction rate. Furthermore, if products do not leave the surface rapidly, they may block reaction sites and thus limit the overall rate. Efficient contacting patterns need to be utilized. Hence, fluidized bed reactors (two-phase backmixed emulator), trickle-bed systems (three-phase packed bed emulator), and slurry reactors (three-phase backmixed emulator) have... 

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