Multiphase fixed-bed reactors

Pangarkar KV. Solving the heat transport issue in multiphase fixed bed reactors [dissertation]. Delft, The Netherlands Delft University of Technology 2010. 

Multiphase fixed bed reactors have complex hydrodynamic and mass transfer characteristics (see also Section 4.9). Thus, the modeling and scale-up are difficult. As an instructive example, we inspect the catalytic 1-octene hydrogenation as a model reaction (Battsengel, Datsevitch, and Jess, 2002 Battsengel, 2002). Table 4.11.4 lists the characteristics of the commercial Ni-catalyst (NISAT, Siidchemie) used for the experiments, data on chemical media, and the parameters that determine the mass transfer. 

Battsengel, B., Datsevitch, L and (ess, A. (2002) Experimental and theoretical studies on hydrogenation in multiphase fixed bed reactors. Chem. Eng. Techn., 25,621-626. 

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. 

The group of Schuth has developed a number of reactors close to conventional testing methods with different degrees of sample integration. For multiphase reactions a 25-fold stirred vessel reactor has been developed [31] and for heterogeneous gas-phase reactions a 16-fold fixed-bed reactor has been presented [32],... 

For more details see Shah (Gas-Liquid-Solid Reactor Design, McGraw-Hill, 1979) and Hofmann [Hydrodynamics and Hydrodynamic Models of Fixed Bed Reactors, in Gianetto and Silveston (eds.), Multiphase Chemical Reactors, Hemisphere 1986]. 

The tubular reactor is a vessel through which the flow is continuous. There are several configurations of tubular reactors suitable for multiphase work, e.g. for liquid-solid and gas-liquid-solid compositions. The flow patterns in these systems are complex. A fixed bed reactor is packed with catalyst, typically formed into pellets of some shape, and if the feed is single phase, a simple tubular plug-flow reactor may suffice (Figure 1.1). Mixed component feeds can be handled in modifications to this. 

FLUIDDYNAMICS, MASS TRANSFER AND CHEMICAL REACTION IN MULTIPHASE CATALYTIC FIXED BED REACTORS... 

This review paper is concentrated on problems in scaling-up multiphase catalytic fixed bed reactors such as trickle-bed or packed bubble column reactors, in which two fluid phases (gas and liquid) pass concurrently through a bed of solid (usually porous) catalyst particles. These types of reactors are widely used in chemical and petrochemical industry as well as in biotechnology and waste water treatment. Typical processes are the hydrodesulphurization of petroleum fractions, the butinediol syntheses in the Reppe process for synthetic rubber, the anthrachinon/hydrochinon process for H202 production, biochemical processes with fixed enzymes or the oxidative treatment of waste water under pressure. 

Whereas in a fixed bed reactor with a single fluid phase there exist only two modes of operation, either downflow (which is used in most cases) or upflow, and only two different flow regimes, either laminar or turbulent flow, which can be observed and characterized by a Reynolds number as the single relevant dimensionless group, the fluiddynamics in multiphase catalytic fixed bed reactors are much more complex. 

Figure 1.2 Exploitable features of membrane reactors, (a) Enhancing the conversion of a reversible reaction in a packed-bed inert membrane reactor, (b) Enhancing the conversion of a reversible reaction in a catalytic membrane reactor, (c) Preventing slip in a reaction requiring stoichiometric feeds, (d) Enhancing the rate of a multiphase reaction, (e) Energetic, thermodynamic, or kinetic coupling of two reactions run on opposite sides of a membrane, (f) Hybrid of fixed-bed reactor (PER) and selective inert membrane reactor (IMR-P) in series. 79...

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. 

Li ZS, Cai NS (2007) ModeUng of multiphase cycles for sorption-enhanced steam methane reforming and sorbent regeneration in fixed bed reactor. Energy Euels 21 2909-2918... 

Esterification of organic compounds often involves multiphase catalytic reactions in which contact of liquid (organic substrate) and solid (catalyst) phases are involved. The most common esterification processes fall into the category of two phase (liquid-solid) reactions. Both slurry and fixed bed reactors can be used for ion exchange resin catalyzed esterification reactions. The overall performance of these reactors depends on the inter phase mass transfer, intrinsic kinetics of reaction, physicochemical properties and mixing of the fluid phases. For a continuous process, fixed bed reactors should be preferred, however, in fixed bed reactors small catalyst particles cause higher pressure drop. Special type of support trays may also be required to support small catalyst particles in fixed bed reactors. 

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. 

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