Commercial FFB Regenerators

The FFB regenerator shown in Fig. 7a is used as the basic regeneration reactor. Coke-laden spent catalyst and recirculated regenerated catalyst enter the bottom of the FFB vessel and are fluidized by the combustion air in the operating regime of fast fluidization. The near regenerated catalyst flows cocurrently with the flue gas out from the top of the FFB vessel. After gas/solid separation, the catalyst falls into the bottom of a disengager situated on top of the FFB vessel and maintained in a mildly fluidized state by air 

This type is selected mainly for grassroots units. It is relatively simple in construction and easy to operate. The principal drawback is the large catalyst inventory in the disengager, which contributes only a few percentage to carbon burning. 

As shown in Fig. 7c, the conventional turbulent bed is used prior to the FFB vessel which is used as the second stage. Combustion air is introduced separately into the two stages. The coke burnt in the FFB vessel is about 10 to 20%. Although the average carbon content of catalyst in the FFB vessel is quite low, the CBI is still higher than ordinary turbulent bed owing to more efficient 02 transfer between phases. There are two modes of CO 

Combustion air and catalyst flow cocurrently and consecutively through two FFB vessels, superimposed one on the other (Chen et al, 1989 Wang et al, 1991), as shown in Fig. 7d. The lower one is an ordinary FFB vessel. Semiregenerated catalyst and flue gas with 4-6% excess 02 leave the lower 

The bayonet tube element is fitted on the outside with a plurality of small diameter tubes or welded with longitudinal fins to enhance heat transmission from the fluidized bed (Zhang et al., 1990 Jiao et al, 1991). The heat flux is normally about 150 to 200 kW/m2, and the heat duty is about 11-22 MW for a typical upflow catalyst cooler with vessel diameter 1.6 to 2.2 m. 

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Typical operating data of the four types of commercial FFB regenerator units are presented in Tables IV, V, VI and VII. These tables show the following distinct features. 

Table VIII presents data on gas and catalyst composition at different axial positions of a commercial FFB regenerator. It can be seen that both carbon content of the catalyst and oxygen content of the gas diminish with height, while C02 shows a general trend of gradual increase.

Fig. 9. Temperature profile of a commercial FFB regenerator, a, circulating catalyst entrance side b, spend catalyst entrance side.

Wei (1990) examined the voidage profiles of some commercial FFB regenerators and fitted those data into the mathematical models proposed by Li et al. (1987, 1988) with good agreement, as shown in Fig. 12. 

The data in Table X show the influence of temperature on CBI and CRC. The data in Table XI show that the calculated CBI based only on chemical kinetics is close to the actual CBI. This is evidence that the chemical reaction rate is the rate controlling step in commercial FFB regenerators. 

Relationship between Outlet Temperature and CBI in a Commercial FFB Regenerator... 

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.

Among the 30 commercial FCCUs with FFB regenerators (including semi-commercial units) now operating in China, regenerator diameter (i.d. 

Operating Data of Commercial FCCUs with Basic FFB Regenerators... 

Operating Data of a Commercial FCCU with 1st Stage FFB Regenerator... 

Certain circumferential temperature differences exist on FFB regenerator from the bottom to the top. The temperature of two catalyst streams entering at the diagonal entrances may differ by as much as 200°C, resulting in a 20 to 50°C measured circumferential bed temperature difference at different elevations. The two curves in Fig. 9 show the axial temperature distribution on two sides of a commercial FFB unit with d, 5 m from 3 m upward from the bottom, indicating a circumferential temperature difference of 20°C, which is evidence of reduced mixing. 

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