Fluidization transport

Whenever the difiusional limitation is broken through the use of fine catalyst powder in a bubbling fluidized bed, a new limitation arises related to the hydrodynamics of the system. In the bubbling fluidized bed, it is not possible to fully exploit the very intrinsic kinetics of the powdered catalyst. Fast fluidization (transport) reactor configuration offers excellent potential to break this limitation. 

Catalytic attrition and entrainment represent a problem in bubbling flui-dized-bed reactor, limiting the range of flow rate and dictates the use of cyclones. In the suggested fast fluidization (transport) reformer, the solid movement is exploited to the maximum limit in an integrated circulating configuration. 

Unpackaged materials in loose, fiee-flowing condition (e.g., powder, granules, pellets, or flakes) which may be stored in silos, bins, tanks, or piles. These materials may be moved by belt conveyors, air-fluidized transport, or bucket conveyors, any of which can cause hazardous dust formation. 

To escape aggregative fluidization and move to a circulating bed, the gas velocity is increased further. The fast-fluidization regime is reached where the soHds occupy only 5 to 20% of the bed volume. Gas velocities can easily be 100 times the terminal velocity of the bed particles. Increasing the gas velocity further results in a system so dilute that pneumatic conveying (qv), or dilute-phase transport, occurs. In this regime there is no actual bed in the column. 

The turbulent fluidized bed has a similar or slightly lower soHds volume fraction than the vigorously bubbling bed. There is considerable transport of soHds out of the turbulent bed and the bed level is not very distinct. Large-scale cyclones are needed to return soHds to the bed. On average, the bed inventory passes through the cyclones several times per hour. 

Transport Disengaging Height. When the drag and buoyancy forces exerted by the gas on a particle exceed the gravitational and interparticle forces at the surface of the bed, particles ate thrown into the freeboard. The ejected particles can be coarser and more numerous than the saturation carrying capacity of the gas, and some coarse particles and clusters of fines particles fall back into the bed. Some particles also coUect near the wall and fall back into the fluidized bed. 

Fresh butane mixed with recycled gas encounters freshly oxidized catalyst at the bottom of the transport-bed reactor and is oxidized to maleic anhydride and CO during its passage up the reactor. Catalyst densities (80 160 kg/m ) in the transport-bed reactor are substantially lower than the catalyst density in a typical fluidized-bed reactor (480 640 kg/m ) (109). The gas flow pattern in the riser is nearly plug flow which avoids the negative effect of backmixing on reaction selectivity. Reduced catalyst is separated from the reaction products by cyclones and is further stripped of products and reactants in a separate stripping vessel. The reduced catalyst is reoxidized in a separate fluidized-bed oxidizer where the exothermic heat of reaction is removed by steam cods. The rate of reoxidation of the VPO catalyst is slower than the rate of oxidation of butane, and consequently residence times are longer in the oxidizer than in the transport-bed reactor. 

Analysis of a method of maximizing the usefiilness of smaH pilot units in achieving similitude is described in Reference 67. The pilot unit should be designed to produce fully developed large bubbles or slugs as rapidly as possible above the inlet. UsuaHy, the basic reaction conditions of feed composition, temperature, pressure, and catalyst activity are kept constant. Constant catalyst activity usuaHy requires use of the same particle size distribution and therefore constant minimum fluidization velocity which is usuaHy much less than the superficial gas velocity. Mass transport from the bubble by diffusion may be less than by convective exchange between the bubble and the surrounding emulsion phase. 

Sasol uses both fixed-bed reactors and transported fluidized-bed reactors to convert synthesis gas to hydrocarbons. The multitubular, water-cooled fixed-bed reactors were designed by Lurgi and Ruhrchemie, whereas the newer fluidized-bed reactors scaled up from a pilot unit by Kellogg are now known as Sasol Synthol reactors. The two reactor types use different iron-based catalysts and give different product distributions. 

Another deep-bed spiral-activated solids-transport device is shown by Fig. ll-60e. The flights cany a heat-transfer medium as well as the jacket. A unique feature of this device which is purported to increase heat-transfer capability in a given equipment space and cost is the dense-phase fluidization of the deep bed that promotes agitation and moisture removal on drying operations. 

The calcination of A1(0H),3 to AI2O3 in a circulating fluidized process produces a high-grade product. The process combines the use of circulating, bubbling, and transport beds to achieve high thermal efficiency. See Fig. 17-28. 

M Mechanical transport of sands to discharge N Noumechauical (gravity or pressure) discharge of underflow S Sedimentation classifier F Fluidized bed classifier... 

Fluidizing systems generally convey prefluidized, finely divided, non-free-flowing materials over short distances, such as from storage bins or transportation vehicles to the entrance of a main conveying... 

FIG. 23-24 Reactors with moving catalysts, a) Transport fluidized type for the Sasol Fischer-Tropsch process, nonregenerating, (h) Esso type of stable fluidized bed reactor/regeuerator for cracldug petroleum oils, (c) UOP reformer with moving bed of platinum catalyst and continuous regeneration of a controlled quantity of catalyst, (d) Flow distribution in a fluidized bed the catalyst rains through the bubbles. 

Several patents exist on carrying out exothermic reactions for manufacture of reactive intermediates where high selectivity is essential. Even this author has a patent to make ethylene oxide in a transport line reactor (Berty 1959). Yet no fluidized bed technology is in use today. Mostly fixed bed, cooled tubular reactors are used for that purpose. 

Fluidized-bed adsorbers have several disadvantages. The continuous handling and transport of solids is expensive from an equipment standpoint fluidized-bed systems must be large to be economical. Solids handling also presents a potential for mechanical problems. Careful control is required to keep the adsorbent fluidized, while minimizing adsorbent loss with the gas-phase attrition of the adsorbent can be high, requiring substantial makeup. 

At any instant, pressure is uniform throughout a bubble, while in the surrounding emulsion pressure increases with depth below the surfaee. Thus, there is a pressure gradient external to the bubble which causes gas to flow from the emulsion into the bottom of the bubble, and from the top of the bubble back into the emulsion. This flow is about three times the minimum fluidization velocity across the maximum horizontal cross section of the bubble. It provides a major mass transport mechanism between bubble and emulsion and henee contributes greatly to any reactions which take place in a fluid bed. The flow out through the top of the bubble is also sufficient to maintain a stable arch and prevent solids from dumping into the bubble from above. It is thus responsible for the fact that bubbles can exist in fluid beds, even though there is no surface tension as there is in gas-liquid systems. 

---