Description of Reactors (Petroleum Refining)
Multiphase catalytic packed-bed reactors (PBRs) operate in two modes: (1) trickle operation, with a continuous gas phase and a distributed liquid phase, and the main mass transfer resistance located in the gas, and (2) bubble operation, with a distributed gas and a continuous liquid phase, and the main mass transfer resistance located in the liquid phase. For three-phase reactions (gas and liquid phases in contact with a solid catalyst), the common modes of operation are trickle- or packed-bed reactors, in which the catalyst is stationary, and slurry reactors, in which the catalyst is suspended in the liquid phase (Figure 2.1). In these reactors, gas and liquid move co-currently down flow or gas is fed countercurrently upflow. Commercially, the former is the most used reactor, in which the liquid phase flows mainly through the catalyst particles in the form of films, rivulets, and droplets (Figure 2.2).
Based on the direction of the fluid flow, PBRs can then be classified as trickle-bed reactors (TBRs) with co-current gas-liquid downflow, trickle-bed reactors with countercurrent
gas-liquid flow, and packed-bubble reactors, where gas and liquid are contacted in co-current upflow. To carry out the catalyst and reactor selection and process design properly, knowledge of what each reactor type can and cannot do is very important. When a fixed-bed reactor is chosen, the question frequently asked is whether to use an upflow or downflow mode of operation.
Figure 2.1. Various types of multiphase catalytic reactors.
Figure 2.2. Liquid flow texture found during the trickle-flow regime in a TBR.
In the case of catalytic packed beds with two-phase flow, such as those used for straight-run naphtha hydrodesulfurization, from a reaction engineering perspective, a large catalyst-to-liquid volume ratio and plug flow of both phases are preferred, and catalyst deactivation is very slow or negligible, which facilitates reactor modeling and design. However, for three -phase catalytic reactors such as those employed for hydrotreating of middle distillates and heavy petroleum fractions, the reaction occurs between the dissolved gas and the liquid-phase reactant at the surface of the catalyst, and the choice of upflow versus downflow operation can be based on rational considerations regarding the limiting reactant at the operating conditions of interest (Dudukovic et al., 2002).
Fixed-Bed Reactors
In a TBR the catalyst bed is fixed (Figure 2.1), the flow pattern is much closer to plug flow, and the ratio of liquid to solid catalyst is small. If heat effects are substantial [i.e., highly exothermic reactions such as those occurring in hydrotreating of unsaturated feeds (l
ight cycle oil from fluid catalytic cracking units)], they can be controlled by recycling of the liquid product stream, although this may not be practical if the product is not relatively stable under reaction conditions or if very high conversion is desired, as in HDS, since recycling causes the system to approach the behavior of a continuous-stirred-tank reactor (CSTR). For such high-temperature increases, the preferred solution is quenching with hydrogen, although the use of other streams has also been reported.Even when a completely vapor-phase reaction in a fixed catalyst bed may be technically feasible, a TBR may be preferred to save energy costs due to reactant vaporization. The limiting reactant may be essentially all in the liquid phase or in both the liquid and gas phases, and the distribution of reactant and products between the gas and liquid phases may vary with conversion.
TBR with Co-current Gas-Liquid Downflow A TBR consists of a column that may be very high (above 10 to 30 m), equipped with one or various fixed beds of solid catalysts, throughout which gas and liquid move in co-current downflow. Figure 2.3 shows the typical film flow texture found during a trickle-flow regime (Gianetto and Specchia, 1992). In this m
ode, gas is the continuous phase and liquid holdup is lower. This operation is the one most used in practice, since there are less severe limitations in throughput than in countercur-rent operation.
For gas-limited reactions (high liquid reactant flux to the catalyst particle, low gas reactant flux to the particle), especially at partially wetted conditions, a downflow reactor is preferred, as it facilitates transport of the gaseous reactant to the catalyst (Dudukovic et al., 2002). In contrast to commercial TBR, in the case of bench-scale TBR operating at equivalent space velocity, the liquid velocity and the catalyst bed length have important effects on the performance of the reactor. The principal advantages and disadvantages of TBR with downflow co-current operation are given below.
Advantages
• Recommended for gas-limited reactions
• Liquid flow approaches plug-flow behavior, which leads to high conversions
Figure 2.3. Nonideal TBR suffering from liquid maldistribution.
reaction mass• Low liquid-solid volume ratio: fewer occurrences of homogeneous side reactions
• Possibility of varying the liquid rate according to catalyst wetting and heat and mass transfer resistances
• A variety of flow regimes allowed; most flexible with respect to varying throughput demands
• The down flow mode also helps keep the bed in place, although with catalysts that are soft or deformable, this might hasten undesired cementation
• Compared with countercurrent flow operation, for co-current flow of the two phases, no limitation on the throughput arises from the phenomenon of flooding, and the quantities of the phase that can be passed depend only on the upstream pressure available because of vaporization effects

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