Fluidized catalyst beds flow features

A salient feature of the fluidized bed reactor is that it operates at nearly constant temperature and is, therefore, easy to control. Also, there is no opportunity for hot spots (a condition where a small increase in the wall temperature causes the temperature in a certain region of the reactor to increase rapidly, resulting in uncontrollable reactions) to develop as in the case of the fixed bed reactor. However, the fluidized bed is not as flexible as the fixed bed in adding or removing heat. The loss of catalyst due to carryover with the gas stream from the reactor and regenerator may cause problems. In this case, particle attrition reduces their size to such an extent where they are no longer fluidized, but instead flow with the gas stream. If this occurs, cyclone separators placed in the effluent lines from the reactor and the regenerator can recover the fine particles. These cyclones remove the majority of the entrained equilibrium size catalyst particles and smaller fines. The catalyst fines are attrition products caused by... 

The H-Cocd Process, based on H-Oil technology, was developed by Hydrocarbon Research, Inc. (HRI). The heart of the process was a three-phase, ebullated-bed reactor in which catalyst pellets were fluidized by the upward flow of slurry and gas through the reactor. The reactor contained an internal tube for recirculating the reaction mixture to the bottom of the catalyst bed. Catalyst activity in the reactor was maintained by the withdrawal of small quantities of spent catalyst and the addition of fresh catalyst. The addition of a catalyst to the reactor is the main feature which distinguishes the H-Coal Process from the typical process. 

The essential feature of a Jluidized-bed reactor is that the solids are held in suspension by the upward flow of the reacting fluid this promotes high mass and heat transfer rates and good mixing. Heat transfer coefficients in the order of 200 W/m-°C between jackets and internal coils are typically obtained. The solids may be a catalyst, a reactant (in some fluidized combustion processes), or an inert powder added to promote heat transfer. 

Whereas for bubbling fluidized beds the solids holdup in the upper part of the reactor and the entrainment of catalyst are often negligible, these features become most important in the case of circulating fluidized beds These systems are operated at gas velocities above the terminal settling velocity ux of a major fraction or even all of the catalyst particles used (% 1 m s 1 < umass flow rales to be externally recirculated are high, up to figures of more than 1000 kg m 2s-1... 

Most fluidized bed partial oxidation processes are operated in the turbulent flow regime of fluidization. However, DuPont operated a circulating fluidized bed catalytic reactor process for maleic anhydride production in Spain, featuring regeneration of the catalyst (by oxidation) on the downcomer side of the circulating system. 

Another type of reactor is the fluidized-bed reactor, in which the powdered catalyst particles are suspended in a stream of up-flowing liquid or gas (Figure 18.9). Another form of this type of reactor is the ebullating-bed reactor (Figure 18.10). The uniform features of these two types of reactor are the efficient mixing of the solid particles (the catalyst) and the fluid (the reactant) that occurs throughout the whole reactor. 

Conventionally, fluid-solid reactions are carried out in various types of reactors, such as packed beds, fluidized/slurry, and monolith reactors as summarized in Table 6.1 [1]. Packed bed reactors are relatively simple, easy to operate and suitable for reactions that require relatively large amounts of catalyst, as they provide a high volumetric catalyst fraction of about 60%. The characteristic feature of packed bed reactors is the pressure drop of the fluid flowing through the catalytic bed. To avoid excessive pressure drop the use of catalyst pellets of 2-6 mm is necessary. But, large porous particles lower the transformation rate through diffusion limitations in the porous network and may decrease product selectivity and yield as discussed in Chapter 2. 

---