Stripping is a mass transfer process. In an analogous fashion to improvements in
distillation performance with better tray designs, the improved methods of
contacting the spent catalyst with steam have made strippers more effective and
lowered the cost of stripper revamps.
Regeneration Technology
The object of the regenerator is to remove the coke that builds up on the catalyst in
the reactor without damaging the catalyst. Many studies have been made on the
burning rates of coke in a fluidized bed of cracking catalyst. Equation 4 describes
the major regeneration operating variables:
dC
dt
¼ K  C i  L m O 2  e
A=RT
:
(4)
The contacting between the oxygen and catalyst is improved significantly as the air
rate or superficial velocity is increased in the regenerator. As the velocity increases,
the bed goes through three stages. A bubbling bed occurs at low superficial
velocities (up to about 1.8 ft/s or 55 cm/s). Here relatively distinct bubbles are
formed and pass through the bed. A turbulent bed (1.8–4.0 ft/s or 55–122 cm/s)
exists at higher superficial velocities in which an emulsion is formed and the
diffusion rate of oxygen is significantly increased. At higher velocities, a fastfluidized bed (4–8 ft/s or 122–244 cm/s) exists in which turbulence is maximized.
A return line from the recovered catalyst to the combustor is required to provide
enough residence time and a sufficient mix temperature for the coke to completely
burn. The difference in burning rates between these idealized regenerator designs is
shown in Fig. 15.
A few observations can be made from Fig. 14 and Eq. 4. At high temperatures
(>1,300
F or 704
C), the burning rate is very high. Oxygen availability limits the
burning rate, though the small size of the fluid cracking catalyst eliminates or
minimizes diffusion as a reaction barrier at normal regenerator temperatures. The
plug flow curve moves up, as catalyst is recycled in the combustor since the effect is
to increase the residence time. Regenerators with larger inventories will reduce the
carbon satisfactorily even if there is a partial malfunction of the air distributor while
smaller inventories allow faster change outs of the catalyst inventory. Over the
years, the regenerator temperatures have increased due to better metallurgy and the
need to burn all of the carbon off the catalyst to restore the zeolite catalysts’ activity
and product selectivity.
At 1,100
F (593
C), the regenerator inventories had to be large to provide
enough time to burn the coke. Full CO combustion raised these temperatures to
1,300–1,350
F (704–732
C). CO promoters are also frequently employed to assist
the carbon burn and prevent the afterburning of CO in the dilute phase or downstream hardware where serious equipment damage can occur. The differences in
280
W. Letzsch
distillation performance with better tray designs, the improved methods of
contacting the spent catalyst with steam have made strippers more effective and
lowered the cost of stripper revamps.
Regeneration Technology
The object of the regenerator is to remove the coke that builds up on the catalyst in
the reactor without damaging the catalyst. Many studies have been made on the
burning rates of coke in a fluidized bed of cracking catalyst. Equation 4 describes
the major regeneration operating variables:
dC
dt
¼ K  C i  L m O 2  e
A=RT
:
(4)
The contacting between the oxygen and catalyst is improved significantly as the air
rate or superficial velocity is increased in the regenerator. As the velocity increases,
the bed goes through three stages. A bubbling bed occurs at low superficial
velocities (up to about 1.8 ft/s or 55 cm/s). Here relatively distinct bubbles are
formed and pass through the bed. A turbulent bed (1.8–4.0 ft/s or 55–122 cm/s)
exists at higher superficial velocities in which an emulsion is formed and the
diffusion rate of oxygen is significantly increased. At higher velocities, a fastfluidized bed (4–8 ft/s or 122–244 cm/s) exists in which turbulence is maximized.
A return line from the recovered catalyst to the combustor is required to provide
enough residence time and a sufficient mix temperature for the coke to completely
burn. The difference in burning rates between these idealized regenerator designs is
shown in Fig. 15.
A few observations can be made from Fig. 14 and Eq. 4. At high temperatures
(>1,300
F or 704
C), the burning rate is very high. Oxygen availability limits the
burning rate, though the small size of the fluid cracking catalyst eliminates or
minimizes diffusion as a reaction barrier at normal regenerator temperatures. The
plug flow curve moves up, as catalyst is recycled in the combustor since the effect is
to increase the residence time. Regenerators with larger inventories will reduce the
carbon satisfactorily even if there is a partial malfunction of the air distributor while
smaller inventories allow faster change outs of the catalyst inventory. Over the
years, the regenerator temperatures have increased due to better metallurgy and the
need to burn all of the carbon off the catalyst to restore the zeolite catalysts’ activity
and product selectivity.
At 1,100
F (593
C), the regenerator inventories had to be large to provide
enough time to burn the coke. Full CO combustion raised these temperatures to
1,300–1,350
F (704–732
C). CO promoters are also frequently employed to assist
the carbon burn and prevent the afterburning of CO in the dilute phase or downstream hardware where serious equipment damage can occur. The differences in
280
W. Letzsch