Application Data Sheet Power Optimizing Catalyst Regeneration with an In Situ Oxygen Probe Optimizing the Performance of the Catalytic Cracking Process Through Better Gas Analysis Catalytic crackers have long been utilized to extract additional gasoline from heavier components resulting from the distillation process. The distillation process is the physical separation of a MIXTURE of different molecules, based upon the different boiling points of these molecules. The catalytic cracking process splits larger hydrocarbon molecules into lighter and higher value components such as gasoline by using a catalyst, which aids the reaction or "cracking" process. The cracking process produces carbon, or coke, which remains on the catalyst particle, reducing its effectiveness over time. Fluidized Catalytic Cracking Units (FCCU) will continuously route coked catalyst into a regenerator unit where oil remaining on the surface of the catalyst is stripped off with steam or solvent. The catalyst is then sent into the regenerator, where air is introduced to burn the coke off of the hot catalyst, usually in suspension. There are many different variations in the regeneration process, including the Continuous Catalyst Regenerator (CCR) process. The regeneration of catalyst frequently becomes a bottleneck that limits the throughput of the catalytic cracking process, so optimizing this process is important. Complete Burn Regeneration A complete burn regeneration process burns all of the coke off, and the resulting flue gases are routed through gas-cleaning equipment, and then to a smokestack. Oxygen is measured in the flue gas resulting from the coke burn-off to maximize the coke removal, and throughput. This may take place at the top of the regeneration tower, and under pressure, or after a turbo- expander that recovers energy and results in lower pressures closer to the smokestack (see Figure 1). In-situ zirconium- oxide oxygen analyzers can be utilized to measure the flue gas O 2 resulting from regeneration in either location. Pressure affects the analyzer readings, so a pressure balancing system is recommended for pressures above 2 PSI. Partial Burn Regeneration A partial burn regeneration process endeavors to volatize, or outgas the coke, and produce CO gas that is then burned in a separate CO boiler. Some coke is also burned in this process, and one goal is to control the CO-to-CO 2 ratio in the regeneration off-gases in order to maximize the burn-off, and hence the throughput. An extractive analytical system utilizing gas chromatographs or Infra-red process analyzers is typically used to measure and control the CO and CO 2 ratios. Refer to Application Note PGC_ANO_Refining_Improving_Catalytic_ Cracking_Vapor for a detailed description of these measuring systems. Oxygen Enrichment Improves Throughput (see Figure 2) Enriching the air used in the regeneration process can increase the coke burn-off rate. Many refineries will mix pure oxygen with the air used for regeneration, resulting in a mixture of 21 to 25 % O 2 , which increases the efficiency of the regeneration process. Figure 1 - Typical Regenerator with Integral Combustor Section O 2 Probe Mounted in Duct Work or Stack To Fractionation Feed Lift Media Air Catalyst Cooler Combustor Riser Hot-Catalyst Circulation Combustor-style Regenerator Open Riser System Flue Gas continued...
Transcript
Application Data Sheet Power
Optimizing Catalyst Regeneration with an In Situ Oxygen Probe
Optimizing the Performance of the Catalytic Cracking Process Through Better Gas AnalysisCatalytic crackers have long been utilized to extract additional gasoline from heavier components resulting from the distillation process. The distillation process is the physical separation of a MIXTURE of different molecules, based upon the different boiling points of these molecules. The catalytic cracking process splits larger hydrocarbon molecules into lighter and higher value components such as gasoline by using a catalyst, which aids the reaction or "cracking" process. The cracking process produces carbon, or coke, which remains on the catalyst particle, reducing its effectiveness over time. Fluidized Catalytic Cracking Units (FCCU) will continuously route coked catalyst into a regenerator unit where oil remaining on the surface of the catalyst is stripped off with steam or solvent. The catalyst is then sent into the regenerator, where air is introduced to burn the coke off of the hot catalyst, usually in suspension. There are many different variations in the regeneration process, including the Continuous Catalyst Regenerator (CCR) process.
The regeneration of catalyst frequently becomes a bottleneck that limits the throughput of the catalytic cracking process, so optimizing this process is important.
Complete Burn RegenerationA complete burn regeneration process burns all of the coke off, and the resulting flue gases are routed through gas-cleaning equipment, and then to a smokestack. Oxygen is measured in the flue gas resulting from the coke burn-off to maximize the coke removal, and throughput. This may take place at the top of the regeneration tower, and under pressure, or after a turbo-expander that recovers energy and results in lower pressures closer to the smokestack (see Figure 1). In-situ zirconium-oxide oxygen analyzers can be utilized to measure the flue gas O2 resulting from regeneration in either location. Pressure affects the analyzer readings, so a pressure balancing system is recommended for pressures above 2 PSI.
Partial Burn RegenerationA partial burn regeneration process endeavors to volatize, or outgas the coke, and produce CO gas that is then burned in a separate CO boiler. Some coke is also burned in this process, and one goal is to control the CO-to-CO2 ratio in the regeneration off-gases in order to maximize the burn-off, and hence the throughput. An extractive analytical system utilizing gas chromatographs or Infra-red process analyzers is typically used to measure and control the CO and CO2 ratios. Refer to Application Note PGC_ANO_Refining_Improving_Catalytic_Cracking_Vapor for a detailed description of these measuring systems.
Oxygen Enrichment Improves Throughput (see Figure 2)Enriching the air used in the regeneration process can increase the coke burn-off rate. Many refineries will mix pure oxygen with the air used for regeneration, resulting in a mixture of 21 to 25 % O2, which increases the efficiency of the regeneration process.
Figure 1 - Typical Regenerator with Integral Combustor Section
O2 Probe Mountedin Duct Workor Stack
To Fractionation
Feed
Lift Media Air
Catalyst Cooler
CombustorRiser
Hot-CatalystCirculation
Combustor-styleRegenerator
Open RiserSystem
Flue Gas
continued...
Figure 4 - Pressure Balanced in Situ O2 Probe with Optional Isolation Valving System (Probe Withdrawn)
Oxymitter Oxygen Probe
Pressure Balancing System
IsolationValve
HART-Wireless capable
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O2 Probe Installation
Spent Catalyst Cooler
Air
Heater
Figure 3 - CCR Regenerator
Oxygen Enrichment Improves ThroughputEnriching the air used in the regeneration process can increase the coke burn-off rate. Many refineries will mix pure oxygen with the air used for regeneration, resulting in a mixture of 22–30 % O2, which increases the speed and throughput of the regeneration process.
The rate of oxygen injection can be controlled by an O2 analyzer just after the mixing valve injection valve (see Figure 1). Pressures are normally 35 PSI, so the probe must be pressure-balanced with reference air.
Continuous Catalytic Regenerator (CCR)Utilized with a "moving bed" process, the CCR employs a regeneration gas loop to control temperature and O2 levels in the regenerator "burn zone." This recirculation loop heats or cools the combustion flue gas resulting from the regeneration process to control temperature, and air is also controlled to an optimum O2 concentration (see Figure 3). The O2 measurement is critical to maintaining optimum rate of regeneration, and for preventing thermal damage inside the burn zone. Process pressures may be close to atmospheric, or pressurized to approximately 35 PSI. Pressure balancing may be required, and an isolation valving system could be needed if probe insertion or withdrawal is required with the process on-line. Pressure balancing will adjust to pressure variations, while traditional O2 systems place a fixed compensation into the electronics. Chlorine is injected into this unit to assist in getting an even coating of platinum or other catalyst materials. A special HCL-resistant sensing cell is required.
Pressure Balanced O2 ProbeMost in situ oxygen analyzers utilize zirconium oxide sensing technology, which is sensitive to pressure variations in the process. An output change of approximately 1 % of reading (not 1 % FS, or 1 % O2) can be expected for every 4'' of H2O pressure in the process. Special accommodation must be made to balance the process pressure with the inside of the oxygen probe. Rosemount Analytical has a special probe design that balances the inside of the probe to the same pressure as the process. A sealed probe is used along with a "booster relay" which duplicates the pressure of the process with the instrument air being used as a reference gas (see figure 4). For accurate, reliable oxygen analysis, Rosemount Analytical offers the 6888 O2 Analyzer with integral electronics. The user friendly design of the probe allows convenient access to internal probe components for in-house service. In addition, the Rosemount Analytical patented electronic cell protection automatically protects the sensor cell's electrodes from harmful corrosive gases. And, the HART® Field Communications Protocol permits all operator functions to be performed from the control room.
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Figure 2 - Typical Regenerator with Separate CO Boiler (Not Shown)
2 StageCyclone
Separators
PlenumChamber
DipLeg
FluidizedCatalyst Bed
Liquid OxygenTank
VaporizationUunit
OxygenAnalyzer
Valve
Flue Gas to CO Boiler
Spent Catalystfrom Reactor
Regenerated Catalyst toReactor-rise
+30 in > H2O Pressure
Air Blower
OxygenAnalyzer
Probe
AirGrid
Spent Catalyst from Reactor
This oxygen measurement can be used for operator information, alarming, or automatic control of the oxygen injection valve (see Figure1). Pressures are normally 35 PSI, so the probe must be pressure-balanced with reference air (see reverse side).
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The Emerson SolutionEmerson Process Management offers a wide array of instrumentation for improving the operation of the catalyst regeneration process including physical measurements (pressure, temperature, and flow), complete analytical solutions, valves and actuation, and SMART Process advanced control solutions. Emerson’s technologies have set the standard for on-line process measurement and control.
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Emerson Process ManagementRosemount Analytical Inc.Analytical Center of Excellence6565P Davis Industrial ParkwaySolon, OH 44139 USAT +1 440 914 1261Toll Free in US and Canada 800 433 6076F +1 440 914 1262US Response Center 800 654 [email protected]
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