Packed catalyst bed

Zabor et al. (Zl) have described studies of the catalytic hydration of propylene under such conditions (temperature 279°C, pressure 3675 psig) that both liquid and vapor phases are present in the packed catalyst bed. Conversions are reported for cocurrent upflow and cocurrent downflow, it being assumed in that paper that the former mode corresponds to bubble flow and the latter to trickle-flow conditions. Trickle flow resulted in the higher conversions, and conversion was influenced by changes in bed height (for unchanged space velocity), in contrast to the case for bubble-flow operation. The differences are assumed to be effects of mass transfer or liquid distribution. 

In a microwave-heated packed catalyst bed, two different forms of hot spots can be created. 

For gas phase heterogeneous catalytic reactions, the continuous-flow integral catalytic reactors with packed catalyst bed have been exclusively used [61-91]. Continuous or short pulsed-radiation (milliseconds) was applied in catalytic studies (see Sect. 10.3.2). To avoid the creation of temperature gradients in the catalyst bed, a single-mode radiation system can be recommended. A typical example of the most advanced laboratory-scale microwave, continuous single-mode catalytic reactor has been described by Roussy et al. [79] and is shown in Figs. 10.4 and... 

In addition to packed catalyst bed, a fluidized bed irradiated by single and multi-mode microwave field, respectively, was also modeled by Roussy et al. [120]. It was proved that the equality of solid and gas temperatures could be accepted in the stationary state and during cooling in a single-mode system. The single-mode cavity eliminates the influence of particle movements on the electric field distribution. When the bed was irradiated in the multimode cavity, the model has failed. Never-... 

The essence of monolithic catalysts is the very thin layers, in which internal diffusion resistance is small. As such, monolithic catalysts create a possibility to control the selectivity of many complex reactions. Pressure drop in straight, narrow channels through which reactants move in the laminar regime is smaller by two or three orders of magnitude than in conventional fixed-bed reactors. Provided that feed distribution is optimal, flow conditions are practically the same across a monolith due to the very high reproducibility of size and surface characteristics of individual monolith passages. This reduces the probability of occurrence of hot spots resulting from maldistributions characteristic of randomly packed catalyst beds. 

Autothermal reforming is a teim adopted for the process in which a mixture of air and steam serves as the oxidant in the conversion of hydrocarbon fuels to a hydrogen-rich product. This process has also been reported to become more efficient as a result of the use of monolithic catalyst beds [11]. An example of this has been the demonstration of a modified version of the fuel-rich partial oxidation process in which noble metal catalysts were used in place of nickel on ceramic monoliths [2j. In earlier reports where packed catalyst beds were used, the concept to control carbon formation, which was predicted by thermodynamic equilibrium at low air-to-fuel ratios, was demonstrated by introducing steam, in addition to air, as an oxidant. 

For the above reasons, countercurrent operation in a packed catalyst bed is seldom applied in practice. One example is the Arosat process [7], where a relatively small catalyst bed is operated in the countercurrent mode at the tail end of a conventional trickle-bed reactor to allow deeper hydrogenation of aromatics in kerosine. Since the absolute amounts of aromatics to be converted is small, little hydrogen is required, and consequently gas velocities can be low, while catalyst utilization is not so important, since the conversion is largely equilibrium limited. A more recent example is the Synsat process [8], which... 

In contrast to packed catalyst beds, however, countercurrent flow of gas and liquid is in principle possible in internally finned monoliths at realistic fluid velocities that are of interest for large-scale industrial applications. The main limitation to countercurrent flow at high velocities is at the outlet of a channel, rather than in the channel itself. With a suitable design of the outlet geometry, however, this problem can be alleviated so that countercurrent operation becomes possible in the velocity range of interest. 

For a packed catalyst bed, the temperature-dependence part of the mass transfer coefficient for a gas-phase reaction can be written as... 

We will also report on experimental work to corroborate our selection, the continuously operated, packed bubble column reactor filled with a packed catalyst bed and with hydrogen feed rates, slightly higher than the stoichiometrically required amount. 

Packed catalyst bed Catalyst pellet Catalyst pellet surface... 

The tubular reactor is a relatively common component on chemical plants. The reactants enter at one end and the products leave from the other, with a continuous variation in the composition and the temperature of the reacting mixture in between. It is common for the feed to consist of a mixture of gases, as in the case of ammonia and methanol synthesis, where the feed gas passes through a densely packed catalyst bed which promotes a number of different reactions simultaneously. 

Mechanical deactivation is due to strong stresses of packed catalysts beds during start ups, shut-downs and catalyst regeneration. 

The experimental arrangement is as described in the previous sections (Fig. 1). The carrier gas flows through the uniformly packed catalyst bed at a flow rate F. The bed itself is very shallow (several millimeters) and the concentration of the sample in the gas phase ((7) at any time t) is to a good approximation uniform throughout the catalyst bed. The axial diffusional spreading in the carrier gas can be made negligibly... 

In a microwave-heated packed catalyst bed, two different forms of hot spot can be created. Hot spots are either macroscopic hot spots (measurable) or microscopic hot spots (on molecular level, immeasurable and similar to those in sonochemistry). 

In a trickle-bed reactor the liquid flows downwards through a packed catalyst bed, while the gas can flow cocurrently or countercurrently to the liquid. The gas phase, which is present in excess, is the continuous phase. In the cocurrent trickle-bed reactor (Fig. 14-8), the gas/liquid mixture leaving the bottom of the reactor is sepa-... 

Figure 4-1 A fixed-bed catalytic reactor with v Kir flowing down through a packed catalyst bed. The design incorporates a number of devices that promote an even distribution of flow through the catalyst bed. (Copyright 2004 UOP LLC. All rights lesraved. Used with permission.)...

---