Turbulent bed regenerator

In comparison, units that are designed with turbulent beds have a lower superficial velocity limit because of soflds entrainment and are unable to independently control the entrained soflds recycle. The soflds loading in the turbulent-bed regenerator configuration are equal to the reactor—regenerator circulation plus the entrained soflds via the cyclone diplegs. 

The overall benefits of this high efficiency combustor over a conventional bubbling- or turbulent-bed regenerator are enhanced and controlled carbon-bum kinetics (carbon on regenerated catalyst at less than 0.05 wt %) ease of start-up and routiae operabiUty uniform radial carbon and temperature profiles limited afterbum ia the upper regenerator section and uniform cyclone temperatures and reduced catalyst iaventory and air-blower horsepower. By 1990, this design was well estabUshed. More than 30 units are ia commercial operation. 

Internal Regenerator Bed Colls. Internal cods generate high overall heat-transfer coefficients [550 W / (m -K)] and typically produce saturated steam up to 4.6 MPa (667 psi). Lower heat fluxes are attained when producing superheated steam. The tube banks are normally arranged horizontally in rows of three or four, but because of their location in a continuously active bubbling or turbulent bed, they offer limited duty flexibdity with no shutdown or start-up potential. 

The work just cited refers to beds of small diameter. Designers and operators of large-scale catalytic fluid beds of Group A powders now appreciate that all of these beds function beyond the Lanneau-Kehoe-Davidson transition (Avidan, 1982 Squires et al., 1985). Most are turbulent beds Sasol reactors and some fluid catalytic cracking regenerators are fast beds. Sasol engineers reported successful development of a turbulent bed for hydrocarbon synthesis (Steynberg et al., 1991). 

In the early days, a conventional single stage turbulent bed (TB) regenerator was used, because the amorphous silica alumina microspheric catalyst used at that time was insensitive to carbon content and its hydrothermal stability was poor. Regeneration temperature was kept at... 

Research and development proceeded for several years and fruitful results were obtained. MPI and SINOPEC have since designed, constructed and operated more than 20 FCCUs with a variety of FFB regenerator configurations. Typical gas velocity was 1.0 to 1.7 m/s, exit temperature 650-730°C, resulting in CBI (in FFB) of 350-650 kg/(t h), which far exceeded that an ordinary turbulent bed. 

The temperature of an FFB regenerator rises gradually from the bottom to the top with an axial temperature gradient of 20-40°C, as compared to less than 10°C for the turbulent bed. The calculated temperature in the bottom zone should be in the neighborhood of 600°C by the assumption of perfect mixing of catalyst and air streams without any backmixing, yet it is far beyond the actual measured temperature. Therefore, ideal plug flow does not exist in the whole FFB, especially in the bottom zone, but the extent of mixed flow is nevertheless much less than that of TB. 

The operating parameters influencing the bed density of an FFB regenerator are somewhat different from those of the turbulent bed. In the latter case, bed density changes only with gas velocity and catalyst inventory, while in the former, solid circulation rate exerts a major influence, as shown in Fig. 10. 

Table XII shows that CBI varies directly with Pe, being low for mixed flow (low Pe) and high for plug flow (high Pe). Commercial FFB regenerators operate between the extremes of complete mixing and plug flow, while the conventional turbulent bed operates essentially with complete mixing. This explains the high efficiency of catalyst regeneration in FFB.

MTO was first scaled up in MRDC s 4 B/D fluid-bed pilot plant in Paulsboro, New Jersey. Following successful completion of the 100 B/D MTG project, the project was extended, and the plant modified to demonstrate MTO (refs. 16, 17). The plant is shown schematically in Fig. 4. Methanol is converted in a turbulent fluid bed reactor with typical conversions exceeding 99.9%. The products are recovered in a simple gas plant. Coked catalyst is continuously withdrawn from the reactor, and the coke is burned in a fluid-bed regenerator. Coke yield and catalyst circulation are an order of magnitude lower than in fluid catalytic cracking. 

The transition to turbulent beds with cat cracking particles can be observed between 0.4 and 0.8 m/s at eunbient conditions and modern regenerators of cat cracking units operate close to or within the turbulent regime. 

The plug-flow model indicates that the fluid velocity profile is plug shaped, that is, is uniform at all radial positions, fact which normally involves turbulent flow conditions, such that the fluid constituents are well-mixed [99], Additionally, it is considered that the fixed-bed adsorption reactor is packed randomly with adsorbent particles that are fresh or have just been regenerated [103], Moreover, in this adsorption separation process, a rate process and a thermodynamic equilibrium take place, where individual parts of the system react so fast that for practical purposes local equilibrium can be assumed [99], Clearly, the adsorption process is supposed to be very fast relative to the convection and diffusion effects consequently, local equilibrium will exist close to the adsorbent beads [2,103], Further assumptions are that no chemical reactions takes place in the column and that only mass transfer by convection is important. 

Maleic anhydride/phthalic anhydride Regenerator bubbling bed/turbulent fluidized bed regime Turbulent fluidized bed regime... 

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. 

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