Diagnosing a problem
Some FCC regenerators operate with a high temperature difference between the flue gas and the dense bed. This is caused by the "afterburn" of the entrained catalyst in the freeboard region. The last post showed that high air flow rate increases this afterburn. Let's see how the system parameters affect the afterburn.
The air distribution parameter, f
As a review, this parameter is the fraction of air that goes to the bubble (or jet) phase at a reference air flow rate. Now you may say, "the operator can increase f by raising the air rate." While it is true that raising the air rate does increase the amount of air to the bubble phase, it does not change that fraction for the reference air flow rate. Only changes in the reactor design, such as air distributors, bed height, catalyst size, can change the fraction at the reference air rate. Compare the plot below with the corresponding plot in the last post. In both plots, the amount of bubble phase gas increases from left to right, but the temperature responses are quite different.
Increasing f decreases both the bed temperature and the flue gas temperature. This is due to the reduction in oxygen available in the emulsion phase for coke burning. The amount of afterburn increases as f increases. At f = 0.3, the afterburn temperature is 35 C.
Exchange rate between emulsion and bubble phases
The plot below shows that increasing the fraction of the bubble phase gas flow that is exchanged with the emulsion phase has little effect on the afterburn or flue gas temperature.
The bed height assumed was 6.5 m. That corresponds to the f2 value of 1 in the plot below. Thus, the abscissa range corresponds to heights from 3.25 to 13 m.
Decreasing the bed height by 50% doubled the afterburn temperature difference from 20 C to 40 C. The smaller bed means less residence time for the catalyst and gas and thus less coke burn in the bed. The entrained catalyst in the freeboard has more coke to burn and also a longer residence time for the burn.
Emulsion phase void fraction
The void fraction in the emulsion phase is primarily determined by the fluidization character of the catalyst. A Geldart Type A powder will have a larger void fraction than either a Type B or D powder.
Increasing the void fraction (i.e., decreasing the catalyst fraction) reduces the coke conversion in the emulsion phase, thereby lowering the bed temperature. The extreme change at the highest void fraction is probably caused by a jump to a lower solution of the emulsion CSTR balances.
The void fraction is expected to be in the lower range so it appears that this variable is not a cause of high afterburn.
The residence time ratio, alpha
This parameter has almost no effect on the temperatures.
From an operations standpoint: excessive air flow can cause high afterburn and high flue gas temperatures (see previous post)
From a system design standpoint: high afterburn may be due to low bed height and/or high gas bypassing (i.e. high f). The latter may be caused by the gas distributor design or the catalyst fluidization properties. Bed height may also affect gas bypassing. This effect will be discussed in the next post.
Next: A CFD simulation of a FCC regenerator