Prior to continuing with turnaround (TA) on the regenerator side, it seemed appropriate to explain the purpose and operation of each zone in the regenerator. I’ll explain how inner screen fouling in the Burn Zone (BZ) depletes O2 in the Chlorination Zone (CZ) and the consequences. I’ll discuss how and when to correctly set the regeneration gas blower flow indication to reliably determine inner screen fouling. I’ll introduce the UOP general operating curve and end with an introduction to the CZ.
For this article I’ll be using the UOP atmospheric regenerator as my reference. Note: UOP ChlorsorbTM will not be covered here.
We concluded CCR Naphtha Reforming Turnarounds III with the Disengaging Hopper (DH) which is positioned above the regenerator in the UOP design. The DH provides surge capacity for the CCR’s catalyst inventory and is integral to catalyst fines removal providing the regenerator with whole catalyst pills and extending the service time of the regenerator inner screen between cleanings.
Where does the burn zone (BZ) start and stop and the CZ begin?
Working with Figure 1, the BZ starts where the perforations begin, at the top of the inner screen, and ends at the interzonal baffle (IZ), highlighted in red. Note: In the UOP CycleMaxTM design, the BZ is laid out similarly but has additional design features that increase coke burning capacity and further preserve catalyst surface area. Regardless of any differences in these two designs, the objective of the BZ is still the same i.e., to burn coke off catalyst prior to its introduction into the CZ. The target coke on catalyst leaving the BZ is < 0.2 wt% with no black centers (split-pill test).
Optimizing the carbon burn step is done by following some fundamental guidelines. UOP has a stated target of adjusting the BZ O2 and catalyst circulation rate to maintain the peak temperature 35 ~ 40% down the length of the BZ. These values are guidelines and were developed by UOP to encourage customers to utilize more of the BZ length to complete the carbon burn.
Figure 2 illustrates an operator maintaining the peak temperature at the 3rd position down the thermocouple bundle, and this may very well correspond with 35 ~ 40% of the BZ length.
The temperature profile shown gives the DCS operator room for the peak temperature to move around as spent catalyst coke shifts from time to time. Example: If coke content on the Rx side goes up and nothing is changed on the regenerator side, the peak temperature will move down the length of the BZ, as it will take longer to complete the burn.
Note: Additional thermocouples can be added to older regenerator designs and will provide much better visibility of the BZ operation. When done, the additional data is frequently viewed as a good return on investment.
If a DCS operator wants to optimize the location of the peak temperature, they have significant flexibility to do so. One can adjust the catalyst circulation rate up or down to provide more or less residence time, adjust the BZ O2 to speed up or slow down the rate of burn, or work with both. Although, the decision an operator makes needs be based on knowing how much coke they’re making on the Rx side and any constraints that may exist on the regenerator side. This will guide the operator to develop the best control strategy for them to use. This concept is a large part of the term “Coke Management”.
When is the carbon burn step complete? Simply put, when coke on catalyst leaving the BZ is < 0.2 wt%. With that, you should also observe little or no activity in the last two TC’s in the BZ. This is a strong indication that coke on catalyst is no longer present and meets the criteria of further processing in the CZ operating at much higher O2 levels. Note: There will be situations when switching between black burn and white burn require a much higher degree of scrutiny to determine that regenerated catalyst has met all the criteria needed to start white burn. This will be discussed in another article that ties all the regenerator zones together.
General Operating Curve:
Figure 3 is referred to as the general operating curve for a UOP CCR PlatformingTM unit. This curve is in every UOP CCR GOM you’ll find and provides guidance on BZ optimization.
With current lab results of coke on spent catalyst and regenerator gas flow rate, one can determine the optimal BZ O2 in mol% with a corresponding catalyst circulation rate. Note: There may be more than one general operating curve in your GOM that covers a different range of BZ O2. Work with your licensor to find out which (or if both) can be used for your regenerator design.
During routine day to day operation this curve is not frequently used, but it’s imperative to understand how to use if you need it
Burn Zone Inner Screen Fouling and Chlorination Zone O2:
With diligent attention to fines removal, the inner screen can maintain 100% flow for several years, but on average the inner screen is pulled for cleaning every 2 years, or so. Once the regen gas flow indication has dropped to 90%, this signals that it’s time to pull and clean the inner screen. So, always plan ahead.
The regen gas flow is generally measured by differential pressure (dP) across the regen gas cooler or across the blower itself.
This flow measurement is typically displayed on the DCS in % and should only be adjusted to read 100% after the inner screen is cleaned, but not until catalyst circulation has been restarted and zone temperatures are back at design.
As the BZ inner screen becomes plugged, the pressure in the BZ will increase. When this happens the IZ baffle dP will also rise from its nominal 5 mm H2O dP (BZ to CZ). This results in an increased flow of depleted O2 flowing from the BZ (0.8 ~ 1.3 mol%) to the CZ that operates at a nominal 15 ~ 18 mol% O2 in an atmospheric regenerator. See Figures 4 and 5. As this takes place, the CZ O2 will drop resulting in poor redispersion of Pt and other metals. This will negatively impact catalyst performance on the Rx side via poor yield structure and reduced catalyst stability i.e., increased rate of catalyst coking.
We’ll make some additional comments on the BZ and related topics in future articles.
During the carbon burn step, coke on catalyst is consumed; although, as a byproduct of the carbon burn step, a significant amount of moisture is manufactured. At the temperatures the BZ operates, Pt and other metals on the catalyst base (CB) are extremely mobile. With that, you’re burning coke off the catalyst, but the metals will no longer be well-dispersed and chloride on catalyst somewhat depleted. Thus, the need for the oxi-chlorination step to follow that will reset the chloride level and catalyst activity, while redispersing metals to optimize yield structure and maintain catalyst stability. This is the same as done during a fixed bed regeneration, but that regeneration is carried out in-situ as opposed to while it’s being circulated continuously (CCR).
In the next article on the CZ we will also discuss the sizing of the CZ restriction orifice, its relationship with the IZ baffle dP and how it’s sized. Moreover, we will take a look at the air-drying zone purpose and correct operation. We welcome your feedback and questions.