Total combustions air = 670 Nm3/h (25000 SCFH) Spent catalyst coke content = 5.0 wt-%
Then from Figure 2
Catalyst circulation rate = 1264 kg/h (2784 pph)
Figure 3.2
3.2 BURN ZONE OXYGEN
The operator controls the Burn Zone oxygen content using the oxygen analyzer 013-AT-502. During normal operation, the oxygen analyzer controls how much excess air is vented from the Regeneration Tower T-1351: as less excess air is vented, more enters the Burn Zone, and the oxygen content becomes higher.
The recommended range for the Burn Zone oxygen is 0.5-1.0 mol% oxygen. Higher oxygen causes higher burn temperatures that could cause damage to the catalyst, primarily in the form of surface area loss. Lower oxygen causes slower coke burning that might not finish completely in the Burn Zone. If coke burning occurs in the Chlorination Zone, extremely high temperatures will occur that will cause serious damage to catalyst, i.e;, alumina phase change and equipment in the Chlorination Zone.
In order to minimize the negative effects of high temperature burning on catalyst performance, it is recommended that the Burn Zone oxygen be minimized, provided that coke burning is completed in the Burn Zone.
There are limits on the acceptable operating range of the oxygen content. These limits are set by the catalyst circulation rate, the spent catalyst coke content, and the Burn Zone gas rate. The minimum allowable oxygen content is determined using the General Operating Curve as shown in the following example:
EXAMPLE 3
Minimum allowable Burn Zone Inlet Oxygen Content If the current Regeneration Section operation is:
Spent catalyst coke content 5.5 wt-%
Burn Zone Flow 110% of design
Catalyst circulation rate 90% of design then from Figure 1:
Minimum allowable Burn Zone oxygen 0.6 mole-%
Using the General Operating Curve, start at 5.5 wt-% on the coke axis. Move vertically to the 110% Burn Zone flow curve. Move horizontally to the left to the vertical line extending up from 90% on the circulation axis. Read 0.6 mole% on the oxygen line.
As a result, if the actual Burn Zone inlet oxygen content is greater than 0.6 mole%, then it could be gradually reduced. If the actual oxygen content is less than 0.6 mole%, then it should be gradually increased.
3.3 SPENT CATALYST COKE 3.3.1 Principle
The operating conditions in the reaction section control the coke on the spent catalyst from the last Platforming reactor. The coke content is a function of charge rate, product octane, charge quality, reactor pressure, recycle rate, and catalyst circulation rate.
The recommended operating range for the spent catalyst coke is 3-7 wt% coke. Within this range, catalyst performance and catalyst life are optimum.
There are limits on the acceptable operating range of the spent catalyst coke content.
These limits are set by the Burn Zone oxygen content, the catalyst circulation rate, and the Burn Zone gas rate. The maximum allowable spent catalyst coke content is determined using the General Operating Curve as shown in the following example:
EXAMPLE 4
Maximum allowable Spent Catalyst Coke Content If the current Regeneration Section operation is:
Catalyst circulation rate 100% of design Burn Zone Inlet oxygen 0.8 mole %
Burn Zone Flow 90% of design
then from Figure 1:
Maximum allowable spent catalyst coke 5.4 wt-%
Using the General Operating Curve, start at 100% on the circulation rate axis. Move vertically to the 0.8 oxygen line. Move horizontally to the right to the 90% Burn Zone flow curve. Then move vertically down to the coke axis. Read 5.4 wt%.
As a result, if the actual spent catalyst coke content is greater than 5.4 wt%, then the current Regeneration Section operation should be adjusted. For example, if the actual spent catalyst coke content is 6 wt%, then one possible adjustment would be to increase the Burn Zone oxygen content. Burn Zone inlet oxygen of 0.85 would be sufficient to combust all the coke in the Burn Zone.
NOTE: The operator should keep the spent catalyst coke content under control.
The spent catalyst coke is laid down on the catalyst in the Platforming reactors. The reactor conditions that increase the laydown rate are:
Increasing charge rate
The spent catalyst coke is burned off in the Regeneration Tower T-1351. The regeneration conditions that increase the burnoff rate are:
Increasing Burn Zone oxygen Increasing Burn Zone gas rate
To keep the spent catalyst coke content under control, the operator should always balance the coke laydown rate and the coke burnoff rate. That means, if a change in reactor conditions increases (or decreases) the coke laydown rate, then the operator must compensate with a change in regeneration conditions that increases (or decreases) the coke burnoff rate. But the change in the regeneration conditions may not need to be made immediately. It may take several days for a change in reactor conditions to show its effect on the spent catalyst coke content. The important point is that the reactor section change eventually shows its effect and then the operator must adjust the regeneration conditions in accordance with the General Operating Curve.
3.3.2 Computation
The operator computes the spent catalyst coke content using two methods: by combustion air usage and by laboratory analysis. The two results should not be different by more than 0.1-0.2 wt% coke.
1. Combustion Air Usage
To compute the coke content by air usage, use the following formula:
X = Y = Burn Zone inlet oxygen concentration, mole fraction AT = total combustion air, Nm3/h (SCFH)
NOTE: For white burn conditions (lower air injection only), use:
AT = AL - VL
Where:
AL = air to Drying Zone, Nm3/h (SCFH) VL = excess air vent, Nm3/h (SCFH) For black burn conditions (upper air injection only), use:
AT = AU Where:
AU = air to Burn Zone, Nm3/h (SCFH)
This formula can be put in chart form for easier use. An example of a typical chart for a 3000 pph Pressurized Regeneration Tower is shown in Figure 2.
EXAMPLE 5