Gas-liquid-solid fixed bed reactor

The second section presents a review of studies concerning counter-currently and co-currently down-flow conditions in fixed bed gas-liquid-solid reactors operating at elevated pressures. The various consequences induced by the presence of elevated pressures are detailed for Trickle Bed Reactors (TBR). Hydrodynamic parameters including flow regimes, two-phase pressure drop and liquid hold-up are examined. The scarce mass transfer data such gas-liquid interfacial area, liquid-side and gas-side mass transfer coefficients are reported. 

Hydrodynamics and mass transfer in fixed-bed gas-liquid-solid reactors operating at high pressure... 

Figure 1-1 Various types of segmented fixed-bed gas-liquid-solid reactor, (a) Horizontal segments of bed, (h) vertical segments of bed, (c) annular segments of bed, (d) catalyst impregnated at the wall.

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). 

The design of a gas-liquid-solid reactor is very much dependent upon the size of the solid particles chosen for the reaction and the anticipated value of the effectiveness factor is one of the most important considerations. Generally, the smaller the particle size the closer the effectiveness factor will be to unity. Particles smaller than about 1 mm in diameter cannot, however, be used in the form of a fixed bed. There would be problems in supporting a bed of smaller particles the pressure drop would be too great and perhaps, above all, the possibility of the interstices between the particles becoming blocked too troublesome. There may, however, be other good reasons for choosing a fixed-bed type of reactor. 

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 types of industrial gas-liquid-solid reactor used in industry can be largely divided into two categories, i.e., one where the solids are fixed and the other where solids are in a suspended state (fluidized bed). Although the choice of the status of the solid depends mainly on the nature of the reaction system, often both fixed- and fluidized-bed systems are examined for the same reaction system (e.g., coal liquefaction). 

A Slurry Bubble Column Reactor (SBCR) is a gas-liquid-solid reactor in which the finely divided solid catalyst is suspended in the liquid by the rising gas bubbles. SBCR offers many advantages over fixed-bed type reactors such as 1) improved heat transfer and mass transfer 2) isothermal temperature profile is maintained and 3) relatively low capital and operating cost. Fischer-Tropsch Synthesis (FTS) takes place in a SBCR where the synthesis gas is converted on catalysts suspended as fine particles in a liquid. The synthesis gas flows in a bubble phase through the catalyst/wax suspension. The volatile products are removed with unconverted gases, and the liquid products are separated firom the suspension. A gas distributor located in the bottom of the reactor produces the bubbles in the reactor. 

Gas-liquid-solid reactors with a trickle-flow regime are the most widely used type of three-phase reactors and are usually operated under steady-state conditions. The behavior of this kind of reactor under the other three-phase fixed-bed reactors is rather complex due to gas and liquid flow concurrently downward through a catalyst packing. For process intensification it is required to improve some of the specific process steps in such chemical reactors. Figure 4.1 shows an overview of different factors that influenced the trickle-bed reactor performance. 

Column reactors for gas-liquid-solid reactions are essentially the same as those for gas-liquid reactions. The solid catalyst can be fixed or moving within the reaction zone. A reactor with both the gas and the liquid flowing upward and the solid circulating inside the reaction zone is called a slurry column reactor (Fig. 5.4-10). The catalyst is suspended by the momentum of the flowing gas. If the motion of the liquid is the driving force for solid movement, the reactor is called an ebullated- or fluidized-bed column reactor (Fig. 5.4-10). When a catalyst is deactivating relatively fast, part of it can be periodically withdrawn and a fresh portion introduced. 

Figure 5.2-1. Types of gas-liquid-solid fixed bed reactors, (a), countercurrent flow (b), cocurrent downflow (c), cocurrent upflow.

The countercurrent-flow fixed-bed operation is often used for physical absorption or for gas-liquid reactions rather than gas-liquid-solid processes. Shah [1] gives a comparison between a gas-liquid-solid (catalytic) fixed bed reactor and a gas-liquid-solid (inert) fixed-bed reactor. The major difference between these two types of reactors are the nature and the size of the packing used and the conditions of gas and liquid flow-rates. 

G. Wild, F. Larachi and A. Laurent, The hydrodynamics characteristics of cocurrent downflow and cocurrent upflow gas-liquid-solid catalytic fixed bed reactors the effect of pressure, Revue de l lnstitut Franfais du Petrole, 46 (1991) 467-490. 

Internal recycle reactors are designed so that the relative velocity between the catalyst and the fluid phase is increased without increasing the overall feed and outlet flow rates. This facilitates the interphase heat and mass transfer rates. A typical internal flow recycle stirred reactor design proposed by Berty (1974, 1979) is shown in Fig. 18. This type of reactor is ideally suited for laboratory kinetic studies. The reactor, however, works better at higher pressure than at lower pressure. The other types of internal recycle reactors that can be effectively used for gas-liquid-solid reactions are those with a fixed bed of catalyst in a basket placed at the wall or at the center. Brown (1969) showed that imperfect mixing and heat and mass transfer effects are absent above a stirrer speed of about 2,000 rpm. Some important features of internal recycle reactors are listed in Table XII. The information on gas-liquid and liquid-solid mass transfer coefficients in these reactors is rather limited, and more work in this area is necessary. 

Table 1-7 Gas-liquid-solid (catalytic) fixed-bed reactor versus ga -liquid-solid (inert) fixed-bed reactor...

The rotating-basket reactor (often known as the Carberry reactor) has been widely used for gas-solid as well as gas-liquid-solid reactions (see Fig. 5-6). Its construction is not very difficult, but it is more complex and expensive to build than a batch or fixed-bed reactor. The catalyst baskets can either be attached to the stirrer [Fig. 5-7(6)] or they can, themselves, be used as the stirrer paddles [Fig. 5-7(a)]. Furthermore, a small variety of rotating catalyst baskets are available (see Fig. 5-8). Baskets must, in general, be small in diameter, so that internal mass-transfer effects are minimized. 

Fixed-bed catalytic reactors are widely applied to reaction systems in which the reactants are present in a single vapor phase. The scale-up and performance of commercial reactors can be predicted from experiments in small-scale reactors. On the other hand, the mixed-phase trickle bed reactor is considerably more complex to analyze and scale up. The performance of trickle bed reactors is influenced by many factors associated with mixed-phase (gas-liquid-solid) processing. Some of... 

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. 

Figure 8.10 Fixed-bed reactors for gas-liquid-solid systems (a) Trickle bed (b) upflow flooded (c) counterflow.

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