Three phase packed bed reactors

The most important competitor to the sluiry reactor for solid/liquid/gas systems is the packed bed reactor operated widi two-phase flow. The main advantage of this type of reactor is that the catalyst remains in the reactor. Mostly porous catalyst particles are used with a size of a few mm. The two-phase flow of gas and liquid can be realized in several ways  

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. 

These columns have two specific advantages for the liquid phase plug flow can be approached, so that high degrees of conversion are possible, and the process stream is easily separated from the catalyst. They are therefore used for processes with not too much heat evolution, where a high degree of conversion is desired, even if the reaction rate is not very high. 

The trickle flow and upflow (bubble flow) operations each have specific advantages. In trickle flow there is less backmixing in the liquid phase. Care should be t Jcen that the solid is completely wetted. When there is a risk of incomplete wetting, and possibly hot spots, upflow may be preferable. Generally, upflow is more suit for relatively low gas and liquid flow rates, which may be related to low reaction rates. Another advantage is the better heat transfer rate at the wall. 

At high flow rates of both phases, pulsed flow occurs, both in upflow and downflow operations, resulting in high pressure drops but also in higher volumetric mass transfer coefficients (Shah and Sharma, 1987). 

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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... 

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. 

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. 

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. 

Son, S. M., Kimura, H., Kusakabe, K. (2011). Esterification of oleic acid in a three-phase, fixed-bed reactor packed with a cation exchange resin catalyst. Bioresource Technology, 102 2), 2130-2132. 

Reactions carried out in three-phase fixed-bed reactors such as hydrogenation, oxidation, and hydrodesulfurization can be highly exothermic. Such situations require incorporation of an efficient heat removal system in order to avoid hot spots or catalyst deactivation as much as possible [13, 92]. A good knowledge of the packed-bed heat transfer parameters is necessary for the design of the reactor and heat removal system. 

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). 

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. 

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... 

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. 

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 ... 

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