Three-phase packed beds

Pressure drop in three-phase packed beds, and thus energy consumption, is a crucial matter in process economics, in particular when large quantities of raw... 

Three-phase packed bed reactors generally have a lower specific capacity than slurry reactors, for two reasons Much larger catalyst particles are used, so that for rapid reactions, with diffusion or mass transfer limitations, much larger catalyst volumes are required. Also, the maximum specific gas/liquid interfacial area is generally smaller. On the other hand, the volumetric mass transfer coefficients at the gas/liquid and at the liquid/solid interfaces are of comparable magnitude, so they are better adapted to one another. Heat transfer rates to the walls are quite limited. 

There are some catalytic processes where a gas and a liquid are contacted with a solid catalyst. They are often carried out in slurry reactors or in three phase packed beds. Also in this case there may be a possibility of dissolving the gaseous reactant in the liquid before it enters the reactor, thus avoiding the problems inherent to gas/liquid mass transfer (example polypropylene process). 

In many solid/liquid/gas systems the liquid phase is the continuous one (section 4.7.1). The other two phases are often dispers, which means that the liquid phase is the "intermediate one that separates the other two. In that case we have either a slurry reactor or a three phase-packed bed reactor. The relative merits of these have been discussed in sections 4,722 and 4,7,2.3, The final choice may be determined by the desired selectivity. When the reaction product tends to undergo a consecutive reaction in the liquid phase, the liquid holdup has to be low (section... 

When a very high degree of conversion of a liquid phase reactant is desired, and the catalyst has a high selectivity, one of the varieties of the three phase packed bed will generally be preferable. When the conversion in the liquid and in the gas have to be high, the two-phase monolith (parallel passage reactor) can be considered. 

Three phase packed bed reactors, filled with catalyst pellets with a size of a few mm. They are usually operated in one of the following ways with the liquid as a continuous phase (cocurrent upflow) or with the liquid running downward in a thin film (triclde flow, cocurrent downward). At high flow rates, pulsating two-phase flow can be obtained. Countercurrent flow is rarely used. 

The relative merits of various ways of operating three phase packed beds have been discussed in section 4.7,2 J. Triclde flow and upflow columns are the most widely... 

When the reaction in the porous catalyst is very rapid, the conversion rate will be determined by gas/liquid or liquid/solid mass transfer. Particularly volumetric gas/liquid mass transfer coefficients (liquid side) are not very much different in slurry-reactors and in three phase packed beds (aU under optimum conditions). 

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... 

The term three-phase fluidization requires some explanation, as it can be used to describe a variety of rather different operations. The three phases are gas, liquid and particulate solids, although other variations such as two immiscible liquids and particulate solids may exist in special applications. As in the case of a fixed-bed operation, both co-current and counter- current gas-liquid flow are permissible and, for each of these, both bubble flow, in which the liquid is the continuous phase and the gas dispersed, and trickle flow, in which the gas forms a continuous phase and the liquid is more or less dispersed, takes place. A well established device for countercurrent trickle flow, in which low-density solid spheres are fluidized by an upward current of gas and irrigated by a downward flow of liquid, is variously known as the turbulent bed, mobile bed and fluidized packing contactor, or the turbulent contact absorber when it is specifically used for gas absorption and/or dust removal. Still another variation is a three-phase spouted bed contactor. 

Bavarian and Fan [3, 4] reported a similar phenomenon occurring in a three-phase fluidized bed. In their case, the hydraulic transport of a packed bed occurred at the start-up of a gas-liquid-solid fluidized bed. Although the cause was different from the case reported in the present study, similar phenomena were observed in both cases. 

In the following equations, the Reynolds number is based on the superficial velocity. Fu and Tan correlation has been derived from experiments conducted in three-phase fixed beds packed with spherical particles and for particle diameters between 0.5 and 1.9 mm. Fixed bed operated under downflow conditions and a liquid distributor was used. The correlation was derived for Rep between 0.1 and 10 (Fu and Tan, 1996) ... 

THREE-PHASE FIXED BEDS TRICKLE-BED AND PACKED BUBBLE-BED REACTORS... 

Three-Phase Fixed Beds Trickle-Bed and Packed Bubble-Bed Reactors... 

In connection with the engineering content of the book, a large number of reactors is analyzed two- and three-phase (slurry) agitated reactors (batch and continuous flow), two-and three-phase fixed beds (fixed beds, trickle beds, and packed bubble beds), three-phase (slurry) bubble columns, and two-phase fluidized beds. All these reactors are applicable to catalysis two-phase fixed and fluidized beds and agitated tank reactors concern adsorption and ion exchange as well. 

Absorption is another gas-liquid process with two phases. Packed bed absorbers have three phases when also taking the solid packing of the column into consideration. Both continuous, i.e., packed bed, and multistage absorption are common in the chemical and biological industry. 

In slurry reactors, an attempt is made to realize intensive and intimate contact between a gas-phase component, usually to be dissolved in the liquid phase, a liquid-phase component and a finely dispersed solid. In this respect, slurry reactors are related to packed-bed reactors with the various gas/liquid flow regimes that can be realized (such as trickle flow, pulsed flow and dispersed bubble flow). Also, there is much similarity with three-phase fluidized beds. 

Typical properties of slurry reactors, and of packed bed co-current downflow trickle flow reactors, are summarized in Table 1. Most properties indicated for slurry reactors also hold for three-phase fluidized beds. These properties can be advantageous or disadvantageous, depending on the application ... 

Gas-liquid-solids reactors Stirred slurry reactors, three-phase fluidized bed reactors (bubble column slurry reactors), packed bubble column reactors, trickle bed reactors, loop reactors. 

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