Total combustions air = 670 Nm3/h (25000 SCFH) Catalyst circulation rate = 1361 kg/h (2784 pph) Then from Figure 2
Spent catalyst coke content = 4.7 wt-%
2. Laboratory Analysis
To commute the coke content form the spent catalyst carbon analysis, use the following formula:
NOTE: Coke is about 95% carbon.
3.4 BURN ZONE GAS FLOW 3.4.1 Principle
The flow of Burn Zone gas is not controlled. The flow rate is the maximum flow rate capable by the regeneration Blower. The rate is a function of the capacity of the Blower and the pressure drop through the Burn Zone loop.
The Burn Zone gas flow rate should be at least 100% of design when the Regenerator screen is clean. Gradually over successive regenerations, the Burn Zone screen plugs with catalyst chips. This decreases the Burn Zone gas flow and the coke burning potential of the Burn Zone. When the Burn Zone gas flow drops to 90% of the rate when the Burn Zone screen is clean, then the screen should be cleaned.
There are limits on the acceptable operating range of the Burn Zone gas rate. These limits are set by the catalyst circulation rate, the Burn Zone oxygen content, and the spent catalyst coke content. The minimum allowable Burn Zone gas rate is determined using the General Operating Curve as shown in the following example:
EXAMPLE 6
Minimum allowable Burn Zone Gas Rate If the current Regeneration Section operation is:
Catalyst circulation rate 95% of design Burn Zone Inlet oxygen 0.6 mole % Spent catalyst coke content 4.5 wt%
then from Figure 1:
Minimum allowable Burn Zone gas rate 93% of design
Using the General Operating Curve, start at 95%% on the circulation rate axis. Move vertically to the 0.6 oxygen line. Move horizontally to the right to the vertical line extending up from 4.5% on the coke axis. Read 93% on the Burn Zone flow curves.
As a result, if the actual Burn Zone gas rate is less than 93% of design, then the current Regeneration Section operation should be adjusted. For example, if the actual Burn Zone gas rate is 90% of design, then one possible adjustment would be to decrease the catalyst circulation rate to the maximum permissible circulation rate than corresponds to 4.5 wt% coke, 90% of design Burn Zone flow, and 0.6 mole% oxygen. This maximum permissible circulation rate is 92% of design. So, the catalyst circulation rate could be decreased from 95% to 92% of design.
3.4.2 Computation
The operator computes the Burn Zone gas flow rate using two methods: by pressure drop across the Regeneration Cooler and by oxygen measurements.
1. Regeneration Cooler
The regeneration Cooler E-1355 causes a pressure drop when Burn Zone gas flows through it. A differential pressure instrument measures this pressure drop. The higher the pressure drop, the greater is the Burn Zone gas flow rate.
From the dimensions of the Cooler on the Vendor’s drawings, a calibration chart is made for computing the flow rate of Burn Zone gas through the Cooler from the measured pressure drop. This chart will be prepared prior to the startup of the unit.
A chart for a typical Regeneration Cooler is shown in Figure 3.3. The actual Burn Zone gas flow rate can then be calculated using this flow and the following formula:
Burn Zone Gas, Nm3/h = Cooler Flow – Vent Gas Flow Where:
Cooler Flow = by direct measurement of Cooler pressure drop and calibration curve, Nm3/h (SCFH)
Vent Gas Flow = approximated by Chlorination Gas Flow, Nm3/h (SCFH)
2. Oxygen Measurements
To compute the Burn Zone gas flow by oxygen measurements, use the following formula:
NOTE : The regeneration Tower T-1351 must be in the black burn condition (upper air injection only).
Figure 3.3
3.5 BURN ZONE BED/REHEAT ZONE BED TEMPERATURES
The operating conditions in the regeneration section affect the “bed” temperatures in the Burn Zone. The “bed” temperatures give a very good indication of the coke burning in the Burn Zone and should be recorded regularly. From top to bottom on the Burn Zone, the “bed” temperatures form a profile. There are three rules concerning the bed profile.
First, the peak temperature should be located about 40% down from the top of the Burn Zone, where the rate of coke burning is greatest. Second, the two Reheat Zone “bed”
temperatures should be the same (i.e., a “flat” profile) because coke burning is essentially complete at the bottom of the Burn Zone. Third, a change in profile alerts the operator to a change in Regeneration Tower T-1351 operating parameters.
The “bed” temperatures are a function of the Burn Zone inlet oxygen concentration, the catalyst circulation rate, the spent catalyst coke, and the regeneration gas rate. The temperatures in the bed rise whenever or wherever the rate of coke burning increases.
The recommended maximum peak “bed” temperature in the Burn Zone is 593°C (1100°F). Higher temperatures could cause damage to catalyst and equipment in the Burn Zone.
The operating conditions that increase the temperature of the peak are:
Increasing Burn Zone oxygen concentration
The operating conditions that lower the location of the peak or increase the temperatures at the bottom of the bed are:
Increasing catalyst circulation rate
Decreasing Burn Zone Oxygen concentration Increasing spent catalyst coke
Decreasing Burn Zone gas rate (by screen plugging)
In order to decrease the rate of surface area decline, it is recommended that the Bun Zone “bed” temperatures be minimized, provided that coke burning is completed in the Burn Zone.