Bed multiphase reactors

Compared to fixed bed multiphase reactors additional modes of operation are available with slurry reactors as also the solid phase can be feeded continuously and eventually recirculated temperature control can be assured by internal cooling coils or/ and external circulation through heat exchangers (for the gas as well as for the liquid phase). Furthermore also partial evaporation and external condensation with recirculation of the liquid phase can support the cooling. 

In this type of reactor, gas superficial velocity is of the order of 0.1-0.3 m/s which is low enough to avoid mechanical interactions between gas and liquid. The velocity of the liquid, in the range 1 - 8.10 m/s is still low, but sufficient to guarantee satisfactory external wetting of the catalyst particles. Table 1 shows advantages and disadvantages of Fixed Bed Multiphase Reactors. Table 2 shows the characteristic parameters of TBRs compared to the two other important multiphase reactors Stirred Slurry and Flooded Fixed Bed Reactor. 

Advantages and Disadvantages of Fixed Bed Multiphase Reactors Advantages Disadvantages... 

Criteria governing the choice between Upflow and Downflow Fixed Bed Multiphase Reactors... 

The packed-bed reactors discussed in Chapters 9 and 10 are multiphase reactors, but the solid phase is stationary, and convective flow occurs only through the fluid phase. The reaction kinetics are pseudohomogeneous, and components balances are written only for the fluid phase. 

Advances in multiphase reactors for fuel industry are discussed in this work. Downer reactors have some advantages over riser reactors, but suffer from some serious shortcomings. The coupled reactors can fully utilize the advantages of the riser and the downer. For fuel industry that involves gas-liquid-solid system, slurry bed reactors especially airlift reactors are preferred due to their performance of excellent heat control and ease of seale up. For high-pressure processes, the spherical reactor is promising due to its special characteristics. 

Naouri et al. (1991) described another contained fluidized bed, the so-called high compacting multiphasic reactor (HCMR), which they used for malic and lactic acid fermentations for wine improvement. Bioparticles were contained within a tapered region and liquid was rapidly recycled through this region by pumping improved liquid/solid contact was cited as the advantage of this reactor. 

Computational fluid dynamics (CFD) is rapidly becoming a standard tool for the analysis of chemically reacting flows. For single-phase reactors, such as stirred tanks and empty tubes, it is already well-established. For multiphase reactors such as fixed beds, bubble columns, trickle beds and fluidized beds, its use is relatively new, and methods are still under development. The aim of this chapter is to present the application of CFD to the simulation of three-dimensional interstitial flow in packed tubes, with and without catalytic reaction. Although the use of... 

A semicontinuous reactor is a reactor for a multiphase reaction in which one phase flows continuously through a vessel containing a batch of another phase. The operation is thus unsteady-state with respect to the batch phase, and may be steady-state or unsteady-state with respect to the flowing phase, as in a fixed-bed catalytic reactor (Chapter 21) or a fixed-bed gas-solid reactor (Chapter 22), respectively. 

This chapter is devoted to fixed-bed catalytic reactors (FBCR), and is the first of four chapters on reactors for multiphase reactions. The importance of catalytic reactors in general stems from the fact that, in the chemical industry, catalysis is the rule rather than the exception. Subsequent chapters deal with reactors for noncatalytic fluid-solid reactions, fluidized- and other moving-particle reactors (both catalytic and noncatalytic), and reactors for fluid-fluid reactions. 

Our treatment of Chemical Reaction Engineering begins in Chapters 1 and 2 and continues in Chapters 11-24. After an introduction (Chapter 11) surveying the field, the next five Chapters (12-16) are devoted to performance and design characteristics of four ideal reactor models (batch, CSTR, plug-flow, and laminar-flow), and to the characteristics of various types of ideal flow involved in continuous-flow reactors. Chapter 17 deals with comparisons and combinations of ideal reactors. Chapter 18 deals with ideal reactors for complex (multireaction) systems. Chapters 19 and 20 treat nonideal flow and reactor considerations taking this into account. Chapters 21-24 provide an introduction to reactors for multiphase systems, including fixed-bed catalytic reactors, fluidized-bed reactors, and reactors for gas-solid and gas-liquid reactions. 

We will develop the rest of this chapter assuming that the catalyst is in a sohd phase with the reactants and products in a gas or liquid phase. In Chapter 12 we will consider some of the more complex reactor types, called multiphase reactors, where each phase has a specific residence time. Examples are the riser reactor, the moving bed reactor, and the transport bed reactor. 

In multiphase reactors we frequently exploit the density differences between phases to produce relative motions between phases for better contacting and higher mass transfer rates. As an example, in trickle bed reactors (Chapter 12) liquids flow by gravity down a packed bed filled with catalyst, while gases are pumped up through the reactor in countercurrent flow so that they may react together on the catalyst surface. 

This shows how catalytic reactions compare with other interfacial reactions. In a fixed bed reactor the catalyst (in phase ) has an infinite residence time, which can be ignored in the expressions we derived in previous chapters. For a moving bed reactor in which catalyst moves through the reactor, we have a true multiphase reactor because the residence time of the catalyst phase is not infinite. 

In this multiphase reactor a tube or tank (a very large tube) is filled with catalyst pellets packed into a bed and a liquid flows down over the catalyst while a gas flows up or down in countercurrent or cocurrent flow. A cross section of this reactor is shown in Figure 12-14. 

A fundamental division of multiphase reactors may be made, depending on whether the solid phase is present as a moving-or as a fixed bed. In principle, one gas-liquid-solid reactor with the fixed bed of solids can be operated in three ways, depending upon the relative orientation of the superficial gas-mass G and superficial liquid-mass L flow-rates (see Figure 5.2-1). 

Knowledge of these types of reactors is important because some industrial reactors approach the idealized types or may be simulated by a number of ideal reactors. In this chapter, we will review the above reactors and their applications in the chemical process industries. Additionally, multiphase reactors such as the fixed and fluidized beds are reviewed. In Chapter 5, the numerical method of analysis will be used to model the concentration-time profiles of various reactions in a batch reactor, and provide sizing of the batch, semi-batch, continuous flow stirred tank, and plug flow reactors for both isothermal and adiabatic conditions. 

Slurry Reactors Slurry reactors are akin to fluidized beds except the fluidizing medium is a liquid. In some cases (e.g., for hydrogenation), a limited amount of hydrogen may be dissolved in the liquid feed. The solid material is maintained in a fluidized state by agitation, internal or external recycle of the liquid using pipe spargers or distributor plates with perforated holes at the bottom of the reactor. Most industrial processes with slurry reactors also use a gas in reactions such as chlorination, hydrogenation, and oxidation, so the discussion will be deferred to the multiphase reactor section of slurry reactors. 

A unified approach has been developed for the prediction of transition in multiphase reactors such as gas-hquid bubble columns, liquid-liquid spray columns, solid-liquid fluidized beds, gas-solid fluidized beds, and three-phase fluidized beds. 

Examples of multiphase reactors (a) trickle-bed reactor, (b) countercurrent packed-bed reactor, (c) bubble column, (d) slurry reactor, and (e) a gas-liquid fluidized bed. [From Reactor Technology by B. L. Taimy. Kirk-Othmer Encyclopedia of Chemical Technology, vol. 19, 3rd ed., Wiley (1982). Reprinted by permission of John Wiley and Sons, Inc., copyright 1982.]... 

As a building block for simulating more complex and practical membrane reactors, various membrane reactor models with simple geometries available from the literature have been reviewed. Four types of shell-and-tube membrane reactor models are presented packed-bed catalytic membrane reactors (a special case of which is catalytic membrane reactors), fluidized-bed catalytic membrane reactors, catalytic non-permselecdve membrane reactors with an opposing reactants geometry and catalytic non-permselective membrane multiphase reactors. Both dense and porous inorganic membranes have been considered. 

Multiphase reactors are reactors in which two or more phases are necessary to carry out the reaction. The majority of multiphase reactors involve gas and liquid phases which contact a solid. In the case of the slurry and trickle bed reactors, the reaction between the gas and the liquid takes place on a solid catalyst Sluface (see Table 12-2). However, in some reactors the liquid phase is an inert medium for the gas to contact the solid catalyst. The latter situation arises when a large heat sink is required for highly exothermic reactions. In many cases the catalyst life is extended by these milder operating conditions. 

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