Understand CO Oxidation Operating Variables to Prevent Afterburn
The prevention of FCC afterburn problems depends on an operators control of those factors which effect CO oxidation.
The means to accomplish the efficient oxidation of carbon monoxide in the FCC regenerator will vary, depending upon unit operating severity.
All refiners take advantage of CO burning in the regenerator to some degree. The variations in CO burn between units are a function most often of differences in either unit metallurgical temperature limitations, other design limitations such as air blower capacity, or other unit operational problems such as a damaged air grid or poor air distribution.
Understanding the relationships between operating variables and CO oxidation rates enables refiners to adjust conditions for maximizing a given unit's effectiveness while preventing afterburn problems.
FCC unit cracking reactions are endothermic making the FCC unit a large consumer of energy. Coke, a by-product of the cracking reaction, must be burned off in the FCC regenerator in order to restore catalyst activity. A substantial amount of heat is transferred to the catalyst during the burning step which raises the FCC catalyst temperature while it is in the regenerator. The regenerated catalyst is circulated back to the reactor bringing with it much of the heat of combustion from the coke burning.
The effective burning of coke from catalyst in the regenerator leads to two key benefits:
1. Improved Catalyst Activity -Figure 1 shows the relationship between coke on regenerated cataIyst (CRC) versus catalyst activity. Improved activity translates into improved yields.
2. Increased Heat to the Reactor- This provides additional heat for the endothermic cracking reaction heat requirements. As shown in Figure 2, roughly two-third's of the potential energy available from the combustion of carbon to CO2 lies in the intermediate step of oxidizing carbon monoxide to carbon dioxide.
Partial Burn Versus Complete Burn Mode of Operation
Carbon burning in an FCC regenerator is classified either as complete or partial. The differences between the two are described by the following equations:
2C + 3/2 O2 ---> CO2 + CO
In a partial burn operation, the CO oxidation is controlled by limiting the amount of oxygen available in the dilute phase or flue gas relative to the dense phase. The temperature rise ( T) or the percent of CO at some point in the flue gas system generally is monitored and used for controlling the air flow rate to the regenerator.
With such a control scheme, any change in reactor conditions which results in higher coke production causes increased oxygen consumption for burning carbon and ultimately reduces the amount of oxygen available for the oxidation of CO to CO2. As a consequence, the controlled after-burning (as reflected by the T measurement) momentarily decreases and this activates controls that automatically increase air rate to the unit in order to restore a preset T value. Once the unit reaches steady state, the T will be maintained at a preset value, the air rate and regenerator temperatures will increase above the base operation. Due to these changes, the catalyst circulation rate will decrease in order to satisfy the constant energy requirement on the reactor side of the unit with hotter catalyst.
In units that monitor CO content in the flue gas the increase in the coke production results in momentary increase in the CO concentration in the flue gas. The increase in the CO above a preset value, forces controls or operators to increase the air rate in order to reduce the carbon monoxide level to the desired level. At steady state, the operating parameters will change as described above.
In the full burn mode of operation, the amount of fuel being burned (carbon and CO) and, therefore, the heat released in the regenerator will vary directly with changes in coke make. When coke production increases, so will the heat released from the regenerator, and conversely a drop in reactor coke make will reduce regenerator heat output.
Maximize CO Oxidation in Regenerator Dense Phase for Afterburn Prevention
As previously shown in Figure 2, both the partial burn and complete burn modes of operation contribute energy to the overall FCC unit heat balance. To maximize the useful utilization of this energy, it is important that to the greatest extent possible, the carbon and CO oxidation should be accomplished in the regenerator dense phase. Doing this takes maximum advantage of the large heat sink potential of the circulating catalyst inventory. Maximizing CO conversion and the subsequent generation and transfer of heat to the catalyst reduces the degree of temperature rise in the upper part of the regenerator where there is relatively little catalyst available to absorb the heat generated from converting CO to CO2.
Failure to maintain and control the oxidation reaction in the dense phase will result in afterburning problems downstream. The higher temperatures caused by afterburning can result in metallurgical damage to downstream equipment.
The key to adjusting temperature rise in the regenerator dilute phase, cyclones, and flue gas lines will be with controlling:
1) the amount of oxygen available for combustion
2) the amount of CO available for combustion
3) the use of carbon monoxide oxidation catalysts such as PROCAT ® and COCAT ® from BASF which preferentially promote the oxidation of CO to CO2 in the regenerator dense phase.
Primary Factors Which Affect CO Oxidation
A summary of all the factors which can affect the oxidation of carbon is given in Figure 3. A more detailed explanation follows below:
A summary of the relationship between regenerator temperature and afterburn problems for units which do not use catalytic CO oxidation promoters is given in Figure 4. From Figure 4, it follows that the lower the dense bed temperature, the more severe the promotion application to prevent after burning. Higher temperatures in the regenerator will increase the rate of reaction of burning of C and CO to CO2.
At low regenerator temperatures (~1100-1150°F), afterburn is more likely to occur farther downstream in the flue gas line. This will happen because the rate of CO burning is slow at low temperatures. Gas travels downstream a considerable distance before reaching a point where the reaction progresses to a level where the heat generated raises temperature to an excessive degree.
At somewhat higher temperaures (~1150-1250°F), the reaction for CO oxidation is faster and excessive temperatures are reached by the time the flue gas gets into the secondary cyclones and flue gas line from the regenerator. If regenerator bed temperature is higher (~1250-1325°F), then the comparatively higher reaction rate will result in the excessive temperatures occurring in the regenerator dilute phase and primary cyclones.
Finally, if regenerator bed temperatures are maintained in the 1325-1400°F range, then the rate of CO oxidation is generally sufficiently fast to insure that all CO burning occurs in the dense bed where the heat sink (catalyst) effectively limits the amount of temperature rise that can occur.
The activity of the regenerated catalyst is directly related to its carbon level. Maximum catalytic activity is attained by minimizing the carbon level. As carbon level on catalyst decreases, so does the rate of carbon oxidation. Therefore, the time required to reduce the coke a set percentage increases as the initial carbon level decreases. The lower the targeted CRC, the more severe the CO promotion requirement.
Increasing the rate of oxygen addition will increase the rate of burn. The oxygen concentration decreases as the air passes through the catalyst to attain the catalyst regeneration targets, therefore, two prerequisites are necessary for CO Oxidation:
1) Sufficient air rate
2) Proper air distribution
Poor air distribution will lead to localized hot spots, poor catalyst regeneration and afterburn problems downstream. Causes of poor air distribution are most often either a damaged air grid or operation outside of the design limits of the air grid. Both can lead to channeling with result that sections of the regenerator can become oxygen starved while excessive oxygen breakthrough into the dilute phase and flue gas line in other sections will lead to afterburning. Insufficient catalyst bed level may also cause channeling and poor air distribution.
In a partial burn operation, channeling will lead to afterburn problems since excessive oxygen remains to react with carbon monoxide. In a full combustion mode, channeling will result in higher CRC levels on the regenerated catalyst.
Higher pressure in the regenerator will increase the oxidation rate due to the increase in the partial pressure of oxygen. Decisions to change pressure must be balanced off against other pressure considerations which include reactor pressure, slide valve differential and superficial velocity within the regenerator.
Metal contaminants in the feedstock tend to deposit on the matrix of FCC catalysts wherein they catalyze the combustion of CO. Generally, only a portion of the total (25-30%) deposited metal is active. Nickel in FCC feed and iron scale are predominant sources of such contaminants. Iron scale in the flue gas lines, cyclones, and dilute phase of the regenerator can be a cause of afterburning problems.
Other metal contaminants such as lead, sodium, and vanadium will act as poisons to the active precious metal contained in CO oxidation promoters. Significant increases in contaminant levels will increase the severity and usage rates for CO promoters.
Antimony and tin are used to passivate the activity of vanadium and nickel on FCC catalyst. Their purpose is to reduce gas make caused by metals catalyzed dehydrogenation. Their effect on CO promotion catalysts is to reduce their activity as well. Thus, the increased use of passivators will increase the severity of the promotion application and the possibility of afterburn problems.
Free silica is that which is found in additive treatment chemicals such as anti-foam suppressants. The effect of free silica is to contaminate CO promotion catalysts thus increasing the severity of operation. When contamination from such sources into the FCC system is suspected, promoter addition rates may have to be increased dramatically.
FCC catalysts which contain higher surface area and trace amounts of contaminant metals will have greater oxidation potential for converting CO to CO2. Conversely, switching to a catalyst with a lower surface area will result in a reduction in catalyst activity for CO conversion.
CO Oxidation Promoter Catalysts
Carbon monoxide combustion promoters are used to enhance the rate of conversion of CO to CO2 in the dense phase of the regenerator. They generally consist of platinum dispersed on a support or carrier. The effectiveness of the promoter depends not solely on the platinum content but upon several factors, including:
1) platinum content and platinum dispersion
2) activity and stability
3) the catalyst support and particle retention characteristics.
Platinum Dispersion, Stability, and Particle Retention Properties are the Key for Promoter Effectiveness
Platinum dispersion is important because higher platinum surface area means more platinum surface available to the CO conversion reaction. BASF draws on its extensive experience in manufacturing precious metals catalysts to ensure excellent platinum dispersion and strong platinum-to-carrier bonding. Figure 5 shows improved CO combustion under standard conditions for PROCAT catalyst versus a competitive promoter that is widely used. This comparison was conducted at equal platinum concentration.
The design of the support is very important in achieving good activity maintenance. In the catalytic cracking reaction, coke is laid down on both the catalyst and promoter surfaces. Coke formed on the surface of the promoter hinders CO combustion promotion in two ways. First, the promoter is not active until it is relatively coke-free, exposing the active metal. Secondly, a coked-up promoter particle also attains much higher particle temperatures as the coke is burned off, which accelerates promoter activity loss. Therefore, the support material should be designed to minimize carbon deposition and must be inert to inhibit catalytic coke formation. The PROCAT catalyst has a very low surface area, and it's matrix is inactive. This reduces coking on the particle, which is particularly important in partial-burn units. Figure 6 compares the difference in coking tendency between the two catalysts. In this laboratory test, fresh promoter was steam deactivated, then run in a MAT unit. Note that the fresh high surface area alumina support produces over three times the coke versus the low surface area silica alumina support. Also, the coking tendency of the low surface area support does not change significantly with steaming severity, indicating the support is inactive.
This is important since the support must be thermally stable to prevent pore collapse leading to a covering of the active precious metal. The support should also play a role in absorbing contaminants and preventing poisons from deactivating the active platinum.
Equally important, the promoter must stay in the unit and must remain in the regenerator dense phase to be effective. An effective promoter should have the coarsest particle size distribution, the highest density, and the greatest attrition resistance possible in order to stay within the dense bed with minimum attrition. This results in maximum effective active metal retention and utilization.