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Process Control Instrumentation
CHAPTER 2
Process Control Instrumentation An FCC unit is a “pressure balance” operation that behaves similarly to a water manometer. Differential pressure between the regenerator and reactor vessels is the driving force that allows for the fluidized catalyst to circulate between the regenerator and reactor vessels. The slide or butterfly valve located in the regenerator flue gas line is used to regulate the differential pressure between the regenerator and reactor vessels. The reactor pressure is controlled by the wet gas compressor (WGC). Fresh catalyst must be added to make up for the catalyst losses from the reactor/regenerator vessels, as well as to compensate for the loss of catalyst activity. The catalyst inventory in the unit is controlled by periodic withdrawal of the excess catalyst from the regenerator vessel. The catalyst level in the regenerator vessel fluctuates and is controlled within a “desirable” level by withdrawal of the catalyst. The catalyst level in the reactor/stripper vessel is controlled by manipulating the spent catalyst slide or plug valve. This slide or plug valve allows enough catalyst to flow into the regenerator in order to maintain the desired catalyst level. Differential pressure indicators across the reactor and regenerator vessels are used to measure the catalyst’s “raw” levels and the catalyst’s flowing densities. In most cat crackers, the flow of “clean” catalyst from the regenerator is automatically regulated via a reactor or riser outlet temperature set point. In very few FCC units, this function is performed manually. In Model IV and Flexicracker designs, the reactor regenerator differential pressure is used to regulate the catalyst circulation rate. In FCC regenerators that operate in complete combustion mode, the total air to the regenerator is adjusted to achieve a desired level of excess oxygen in the regenerator flue gas. The regenerator bed temperature often fluctuates and it is manually adjusted by manipulating feed quality, preheat temperature, the use of recycle streams to the riser, stripping steam rate, and possible adjustments to the fresh catalyst addition rate and/or activity. In partial burn mode of catalyst regeneration, the regenerator temperature and carbon on the catalyst are often controlled by regulating air rate to the regenerator and/or targeting a desired concentration of CO in the regenerator flue gas. Fluid Catalytic Cracking Handbook. © 2012 Elsevier Inc. All rights reserved.
43
44
Chapter 2
Operating Variables The key operating parameters in the reactor regenerator section include the following: • • • • • • • • • • • •
Fresh feed rate LCO, HCO, or slurry recycle to the riser Riser outlet or reactor cyclones outlet temperature Feed preheat temperature Reactor and/or regenerator pressures Flue gas excess oxygen CO concentration of regenerator flue gas (partial combustion) Regenerator dense bed temperature (partial combustion) Coke on regenerated catalyst (partial combustion) Stripping steam rate Feed nozzle atomization steam Catalyst addition rate or fresh catalyst surface area.
Process Control Instrumentation Process control instrumentation controls the FCC unit in a safe, monitored mode with limited operator intervention. Two levels of process control are used: • •
Basic supervisory control Advanced process control (APC).
Basic Supervisory Control The primary controls in the reactor regenerator section are flow, temperature, pressure, and catalyst level. The flow controllers are often used to set desired flows for the fresh feed, recycle, air rate, stripping steam, dispersion steam, and so on. Each flow controller usually has three modes of control: manual, auto, and cascade. See Figure 2.1 for a typical process flow diagram (PFD). In manual mode, the operator manually opens or closes a valve to the desired percent opening. In auto mode, the operator enters the desired flow rate as a set point. In cascade mode, the controller set point is an input from another controller. The reactor temperature is controlled by a temperature controller that regulates the regenerated catalyst slide valve. The regenerator temperature is not automatically controlled but depends on its mode of catalyst regeneration. In partial combustion, the regenerator
Air preheater
Air compressor
Regenerator
Reactor
Main fractionator
FV FT
Stripping steam MF OVHD air cooler
PT
PDT
MF cooler
Flare
Flue gas TT
Flue gas slide valve
Reflux Cat naphtha
MF accum
PT
LCO
Riser
PT
Slurry oil WGC KO drum
LI
Torch oil
Steam Feed LV FV
Air preheater
Wet gas compressor
FT
Figure 2.1: Typical FCC unit process flow diagram (PFD) (FV 5 flow control valve, FT 5 flow transmitter, KO 5 knock out, LI 5 level indicator, LV 5 level control valve, MF 5 main fractionator, OVHD 5 overhead, PDT 5 pressure differential transmitter, PT 5 pressure transmitter, TV 5 temperature control valve).
Process Control Instrumentation 45
TV
Air
46
Chapter 2
temperature is controlled by adjusting the flow of air to the regenerator. In full burn, the regenerator temperature is a function of several variables, including feedstock quality, catalyst properties, use of recycle, stripping steam rate, and mechanical conditions of the feed injection system and the catalyst stripper. The reactor pressure is not directly controlled, instead it floats on the main column overhead receiver. A pressure controller on the overhead receiver controls the WGC and indirectly controls the reactor pressure. The regenerator pressure is often controlled directly by regulating the flue gas slide or butterfly valve. (In some cases, the flue gas slide or butterfly valve is used to control the differential pressure between the regenerator and reactor.) The reactor or stripper catalyst level is maintained with a level controller that regulates the movement of the spent catalyst slide valve. The regenerator level is manually controlled to maintain catalyst inventory. Regenerated and Spent Catalyst Slide Valve Low Differential Pressure Override
Normally, the reactor temperature and the stripper level controllers regulate the movement of the regenerated and spent catalyst slide valves. The algorithm of these controllers can drive the valves either fully open or fully closed if the controller set point is unobtainable. It is extremely important that a positive and stable pressure differential be maintained across both the regenerated and spent catalyst slide valves. For safety, a low differential pressure controller overrides the temperature/level controllers, should these valves open too much. The shutdown is usually set at 2 psi (14 kPa). An example of a typical shutdown matrix is shown in Table 2.1. The direction of the catalyst flow must always be from the regenerator to the reactor and from the reactor back to the regenerator. A negative differential pressure across the regenerated catalyst slide valve can allow hydrocarbons to backflow into the regenerator. This is called a “flow reversal” and can result in an uncontrolled afterburn and possible equipment damage. A negative pressure differential across the spent catalyst slide valve can allow air to backflow from the regenerator into the reactor with equally disastrous consequences. To protect the reactor and the regenerator against a flow reversal, pressure differential controllers (PDICs) are used to monitor and control the differential pressures across the slide valves. If the differential pressure falls below a minimum set point, the PDIC overrides the process controller and closes the valve. Only after the PDIC is satisfied will the control of the slide valve return to the process.
Process Control Instrumentation 47 Table 2.1:
Typical Shutdown Matrix.
Cause
RCSV
Riser Emergency Steam
Feed to Riser
Slurry Recycle
HCO Recycle
SCSV
Regenerator Emergency Steam
Normal RCSV low differential pressure RCSV low/low differential pressure SCSV low differential pressure SCSV low/low differential pressure Air blower low/low air flow Riser low/low feed flow Low reactor temperature Reactor/stripper high catalyst level Manual shutdown
Process
Closed
Process
Process
Process
Process
Closed
Alarm Only
X
Close
Open
Close
Close
Close
X
Close
Close
Open
Close
Open
Close
Close
Close
Open
Open
Close
X
Close
Open
Close
Close
Close
Close
Open
Close
Close
Close
X Close
Open
RCSV 5 regenerated catalyst slide valve; SCSV 5 spent catalyst slide valve.
Advanced Process Control To maximize the unit’s profit, one must operate the unit simultaneously against as many constraints as possible. Examples of these constraints are limits on the air blower, WGC, reactor/regenerator temperatures, slide valve differentials, and so on. The conventional regulatory controllers work only one loop at a time and they do not talk to one another. A skilled operator can “push” the unit against more than one constraint at a time, but the constraints often change. To operate closer to multiple constraints, a number of refiners have installed an APC package either within their distributed control system (DCS) or in a host computer. The primary advantages of an APC are as follows: •
It provides more precise control of the operating variables against the unit’s constraints and therefore obtains incremental throughput or cracking severity.
48 • •
Chapter 2 It is able to respond quickly to ambient disturbances, such as cold fronts or rainstorms. It can run a day/night operation, taking advantage of the cooler temperatures at night. It pushes against two or more constraints rather than one single constraint. It can maximize the air blower and WGC capacities.
As mentioned above, there are two options for installing an APC. One option is to install an APC within the DCS framework, and the other is to install a multivariable modeling/control package in a host computer. Each has advantages and disadvantages, as indicated below. Advantages of Multivariable Modeling and Control The multivariable modeling/control package is able to hold more tightly against constraints and recover more quickly from disturbances. This results in an incremental capacity used to justify multivariable control. An extensive test run is necessary to measure the response of unit variables. In APC on a DCS framework, the control structure has to be designed, configured, and programmed for each specific unit. Modifying the logic can be an agonizing process. Wiring may be necessary. It is difficult to document the programming and difficult to test. With a host computer framework, the control package is all in the software. Changing the program can still be agonizing, but the program can be tested off-line. There is more flexibility in the computer system, which can be used for many other purposes, including online heat and weight balances. Disadvantages of Multivariable Modeling and Control A multivariable model is like a “black box.” The constraints go in and the signals come out. Operators do not trust a system that takes the unit away from them. Successful installations require good training and continual communication. The operators must know the interconnections in the system. The model may need expensive work if changes are made during a turnaround. If the feed gets outside the range the unit was modeled for, results can be at best unpredictable. An upset can happen for which the system was not programmed. The DCS-based APC is installed in a modular form, meaning operators can understand what the controlled variable is tied to a little more easily. The host computer-based system may have its own problems, including computer-tocomputer data links. In any APC the operator has to be educated and brought into it before he or she can use it. The control has to be properly designed, meaning the model has to be configured and “tuned” properly. The operators need to be involved early and all of them need to be consulted. All four shifts may be running the unit differently.
Process Control Instrumentation 49
Summary In most FCC units, the instrumentations that are shown in the piping and instrumentation diagrams (P&IDs) are often the minimum needed to operate the FCC unit. Many FCC units do not take advantage of DCS capabilities for efficient and reliable operations of the cat cracker. Instrument diagnostics can be used to detect accuracy and status of the transmitters. These diagnostics features can alert console operators with the accuracy of the measuring process variables, such as catalyst level, slide valve differentials, and cracking temperatures (see Table 2.1). DCS screens can be configured to display items such as cyclone velocities, cyclone pressure drops, actual catalyst bed levels, rate-of-change alarms, regenerator superficial velocity, and many other parameters. An APC package (whether within the DCS framework or as a host-based multivariable control system) provides more precise control of operating variables against the unit’s constraints. It will gain incremental throughput or cracking severity. A properly designed APC operates the unit safely and yet continually, while optimizing feed rate, operating severity, product qualities, and environmental controls, as well as staying within the unit’s constraints.
Process Control Instrumentation An FCC unit is a “pressure balance” operation that behaves similarly to a water manometer. Differential pressure between the regenerator and reactor vessels is the driving force that allows for the fluidized catalyst to circulate between the regenerator and reactor vessels. The slide or butterfly valve located in the regenerator flue gas line is used to regulate the differential pressure between the regenerator and reactor vessels. The reactor pressure is controlled by the wet gas compressor (WGC). Fresh catalyst must be added to make up for the catalyst losses from the reactor/regenerator vessels, as well as to compensate for the loss of catalyst activity. The catalyst inventory in the unit is controlled by periodic withdrawal of the excess catalyst from the regenerator vessel. The catalyst level in the regenerator vessel fluctuates and is controlled within a “desirable” level by withdrawal of the catalyst. The catalyst level in the reactor/stripper vessel is controlled by manipulating the spent catalyst slide or plug valve. This slide or plug valve allows enough catalyst to flow into the regenerator in order to maintain the desired catalyst level. Differential pressure indicators across the reactor and regenerator vessels are used to measure the catalyst’s “raw” levels and the catalyst’s flowing densities. In most cat crackers, the flow of “clean” catalyst from the regenerator is automatically regulated via a reactor or riser outlet temperature set point. In very few FCC units, this function is performed manually. In Model IV and Flexicracker designs, the reactor regenerator differential pressure is used to regulate the catalyst circulation rate. In FCC regenerators that operate in complete combustion mode, the total air to the regenerator is adjusted to achieve a desired level of excess oxygen in the regenerator flue gas. The regenerator bed temperature often fluctuates and it is manually adjusted by manipulating feed quality, preheat temperature, the use of recycle streams to the riser, stripping steam rate, and possible adjustments to the fresh catalyst addition rate and/or activity. In partial burn mode of catalyst regeneration, the regenerator temperature and carbon on the catalyst are often controlled by regulating air rate to the regenerator and/or targeting a desired concentration of CO in the regenerator flue gas. Fluid Catalytic Cracking Handbook. © 2012 Elsevier Inc. All rights reserved.
43
44
Chapter 2
Operating Variables The key operating parameters in the reactor regenerator section include the following: • • • • • • • • • • • •
Fresh feed rate LCO, HCO, or slurry recycle to the riser Riser outlet or reactor cyclones outlet temperature Feed preheat temperature Reactor and/or regenerator pressures Flue gas excess oxygen CO concentration of regenerator flue gas (partial combustion) Regenerator dense bed temperature (partial combustion) Coke on regenerated catalyst (partial combustion) Stripping steam rate Feed nozzle atomization steam Catalyst addition rate or fresh catalyst surface area.
Process Control Instrumentation Process control instrumentation controls the FCC unit in a safe, monitored mode with limited operator intervention. Two levels of process control are used: • •
Basic supervisory control Advanced process control (APC).
Basic Supervisory Control The primary controls in the reactor regenerator section are flow, temperature, pressure, and catalyst level. The flow controllers are often used to set desired flows for the fresh feed, recycle, air rate, stripping steam, dispersion steam, and so on. Each flow controller usually has three modes of control: manual, auto, and cascade. See Figure 2.1 for a typical process flow diagram (PFD). In manual mode, the operator manually opens or closes a valve to the desired percent opening. In auto mode, the operator enters the desired flow rate as a set point. In cascade mode, the controller set point is an input from another controller. The reactor temperature is controlled by a temperature controller that regulates the regenerated catalyst slide valve. The regenerator temperature is not automatically controlled but depends on its mode of catalyst regeneration. In partial combustion, the regenerator
Air preheater
Air compressor
Regenerator
Reactor
Main fractionator
FV FT
Stripping steam MF OVHD air cooler
PT
PDT
MF cooler
Flare
Flue gas TT
Flue gas slide valve
Reflux Cat naphtha
MF accum
PT
LCO
Riser
PT
Slurry oil WGC KO drum
LI
Torch oil
Steam Feed LV FV
Air preheater
Wet gas compressor
FT
Figure 2.1: Typical FCC unit process flow diagram (PFD) (FV 5 flow control valve, FT 5 flow transmitter, KO 5 knock out, LI 5 level indicator, LV 5 level control valve, MF 5 main fractionator, OVHD 5 overhead, PDT 5 pressure differential transmitter, PT 5 pressure transmitter, TV 5 temperature control valve).
Process Control Instrumentation 45
TV
Air
46
Chapter 2
temperature is controlled by adjusting the flow of air to the regenerator. In full burn, the regenerator temperature is a function of several variables, including feedstock quality, catalyst properties, use of recycle, stripping steam rate, and mechanical conditions of the feed injection system and the catalyst stripper. The reactor pressure is not directly controlled, instead it floats on the main column overhead receiver. A pressure controller on the overhead receiver controls the WGC and indirectly controls the reactor pressure. The regenerator pressure is often controlled directly by regulating the flue gas slide or butterfly valve. (In some cases, the flue gas slide or butterfly valve is used to control the differential pressure between the regenerator and reactor.) The reactor or stripper catalyst level is maintained with a level controller that regulates the movement of the spent catalyst slide valve. The regenerator level is manually controlled to maintain catalyst inventory. Regenerated and Spent Catalyst Slide Valve Low Differential Pressure Override
Normally, the reactor temperature and the stripper level controllers regulate the movement of the regenerated and spent catalyst slide valves. The algorithm of these controllers can drive the valves either fully open or fully closed if the controller set point is unobtainable. It is extremely important that a positive and stable pressure differential be maintained across both the regenerated and spent catalyst slide valves. For safety, a low differential pressure controller overrides the temperature/level controllers, should these valves open too much. The shutdown is usually set at 2 psi (14 kPa). An example of a typical shutdown matrix is shown in Table 2.1. The direction of the catalyst flow must always be from the regenerator to the reactor and from the reactor back to the regenerator. A negative differential pressure across the regenerated catalyst slide valve can allow hydrocarbons to backflow into the regenerator. This is called a “flow reversal” and can result in an uncontrolled afterburn and possible equipment damage. A negative pressure differential across the spent catalyst slide valve can allow air to backflow from the regenerator into the reactor with equally disastrous consequences. To protect the reactor and the regenerator against a flow reversal, pressure differential controllers (PDICs) are used to monitor and control the differential pressures across the slide valves. If the differential pressure falls below a minimum set point, the PDIC overrides the process controller and closes the valve. Only after the PDIC is satisfied will the control of the slide valve return to the process.
Process Control Instrumentation 47 Table 2.1:
Typical Shutdown Matrix.
Cause
RCSV
Riser Emergency Steam
Feed to Riser
Slurry Recycle
HCO Recycle
SCSV
Regenerator Emergency Steam
Normal RCSV low differential pressure RCSV low/low differential pressure SCSV low differential pressure SCSV low/low differential pressure Air blower low/low air flow Riser low/low feed flow Low reactor temperature Reactor/stripper high catalyst level Manual shutdown
Process
Closed
Process
Process
Process
Process
Closed
Alarm Only
X
Close
Open
Close
Close
Close
X
Close
Close
Open
Close
Open
Close
Close
Close
Open
Open
Close
X
Close
Open
Close
Close
Close
Close
Open
Close
Close
Close
X Close
Open
RCSV 5 regenerated catalyst slide valve; SCSV 5 spent catalyst slide valve.
Advanced Process Control To maximize the unit’s profit, one must operate the unit simultaneously against as many constraints as possible. Examples of these constraints are limits on the air blower, WGC, reactor/regenerator temperatures, slide valve differentials, and so on. The conventional regulatory controllers work only one loop at a time and they do not talk to one another. A skilled operator can “push” the unit against more than one constraint at a time, but the constraints often change. To operate closer to multiple constraints, a number of refiners have installed an APC package either within their distributed control system (DCS) or in a host computer. The primary advantages of an APC are as follows: •
It provides more precise control of the operating variables against the unit’s constraints and therefore obtains incremental throughput or cracking severity.
48 • •
Chapter 2 It is able to respond quickly to ambient disturbances, such as cold fronts or rainstorms. It can run a day/night operation, taking advantage of the cooler temperatures at night. It pushes against two or more constraints rather than one single constraint. It can maximize the air blower and WGC capacities.
As mentioned above, there are two options for installing an APC. One option is to install an APC within the DCS framework, and the other is to install a multivariable modeling/control package in a host computer. Each has advantages and disadvantages, as indicated below. Advantages of Multivariable Modeling and Control The multivariable modeling/control package is able to hold more tightly against constraints and recover more quickly from disturbances. This results in an incremental capacity used to justify multivariable control. An extensive test run is necessary to measure the response of unit variables. In APC on a DCS framework, the control structure has to be designed, configured, and programmed for each specific unit. Modifying the logic can be an agonizing process. Wiring may be necessary. It is difficult to document the programming and difficult to test. With a host computer framework, the control package is all in the software. Changing the program can still be agonizing, but the program can be tested off-line. There is more flexibility in the computer system, which can be used for many other purposes, including online heat and weight balances. Disadvantages of Multivariable Modeling and Control A multivariable model is like a “black box.” The constraints go in and the signals come out. Operators do not trust a system that takes the unit away from them. Successful installations require good training and continual communication. The operators must know the interconnections in the system. The model may need expensive work if changes are made during a turnaround. If the feed gets outside the range the unit was modeled for, results can be at best unpredictable. An upset can happen for which the system was not programmed. The DCS-based APC is installed in a modular form, meaning operators can understand what the controlled variable is tied to a little more easily. The host computer-based system may have its own problems, including computer-tocomputer data links. In any APC the operator has to be educated and brought into it before he or she can use it. The control has to be properly designed, meaning the model has to be configured and “tuned” properly. The operators need to be involved early and all of them need to be consulted. All four shifts may be running the unit differently.
Process Control Instrumentation 49
Summary In most FCC units, the instrumentations that are shown in the piping and instrumentation diagrams (P&IDs) are often the minimum needed to operate the FCC unit. Many FCC units do not take advantage of DCS capabilities for efficient and reliable operations of the cat cracker. Instrument diagnostics can be used to detect accuracy and status of the transmitters. These diagnostics features can alert console operators with the accuracy of the measuring process variables, such as catalyst level, slide valve differentials, and cracking temperatures (see Table 2.1). DCS screens can be configured to display items such as cyclone velocities, cyclone pressure drops, actual catalyst bed levels, rate-of-change alarms, regenerator superficial velocity, and many other parameters. An APC package (whether within the DCS framework or as a host-based multivariable control system) provides more precise control of operating variables against the unit’s constraints. It will gain incremental throughput or cracking severity. A properly designed APC operates the unit safely and yet continually, while optimizing feed rate, operating severity, product qualities, and environmental controls, as well as staying within the unit’s constraints.