Advanced Control Strategies
The design includes the use of Advanced Regulatory Control (ARC), Inferential Advanced Control, and Multi-Variable Constraint Predictive (MPC) Control ACSC Propriety Software.
Advanced Regulatory Control (ARC):
The first level of Advanced Process Control (APC) is Advanced Regulatory Control (ARC) functions. which are designed to reduce the effect of plant disturbances on critical process variables. The design includes the use of Advanced Regulatory Control (ARC) on complex loops.
MPC-Based Advanced Controls:
The next level includes the use of Multi-variables Predictive Constraint (MPC) control on selected loops Additional benefits will result from applying MPC to complex loops. These benefits are due largely to smoother operation and the constraint handling capability of the MPC controller which results in the ability to run the unit at true constraint, rather than having to operate at some safe distance from constraint in anticipation of large upsets. Further benefits are derived from the ability of the controller to decouple all interactions simultaneously, which translates into improved unit stability and reduced variability in product qualities. Also, due to the linear programming embedded in the controller, the controller can optimize the tradeoff between improving separation and product yields and conserving energy. The following MPC controllers will be used to implement identified MVC-based applications.
· A single sub-MPC controller is selected for the fuel gas stripper and deethanizer control.
· A single sub-MPC controller is selected for the depropanizer control
· A single sub-MPC controller is selected for the debutanizer control
· A single sub-MPC controller is selected for the depentanizer/hexane control
· An overall-plant MPC controller is selected to coordinate the operation of the sub-controllers and optimize the operation of the unit. If there is excess capacity maximize the unit throughput to the unit.
The Fuel Gas Stripper ARC includes:
· Surge Volume Control (SVC) on the tower bottom level cascading onto the tower bottom flow.
· Surge Volume Control (SVC) on the overhead condenser accumulator cascading onto the condenser refrigerant level to control the refrigerant flow to the condenser. A high overhead accumulator level overrides the reflux drum LC to control the refrigerant flow.
· Tower overhead pressure cascading onto fuel gas flow.
· Reflux-to-feed ratio.
· Reboiler heat duty-to-feed ratio.
The Deethanizer ARC includes:
· Surge Volume Control (SVC) on the tower bottom cascading onto the tower bottoms flow.
· Surge Volume Control (SVC) on the overhead condenser level cascading onto the condenser refrigerant flow. A high overhead accumulator level overrides the reflux condenser LC to control the refrigerant flow.
· Tower overhead pressure control cascading onto overhead flow to the acetylene converter.
· Reflux-to-feed ratio.
· Reboiler heat duty-to-feed ratio.
The Depropanizer ARC includes:
· Surge Volume Control (SVC) on the tower bottom cascading onto the tower bottoms flow.
· Surge Volume Control (SVC) on the overhead condenser accumulator cascading onto the overhead reflux.
· Tower overhead pressure cascading onto overhead ethane flow to the cryogenic.
· Tower overhead to feed ratio control.
· Tower reboiler duty to feed ratio control.
· Top differential control.
The Debutanizer ARC includes:
· Surge Volume Control (SVC) on the tower bottom cascading onto the tower bottoms flow.
· Surge Volume Control (SVC) on the overhead condenser accumulator cascading onto the overhead reflux.
· Tower overhead pressure control cascading onto overhead vapor flow.
· Overhead reflux-to-feed ratio.
· Reboiler Duty-to-feed ratio.
The Depentanizer/Dehexanizer ARC includes:
· Surge Volume Control (SVC) on the tower bottom cascading onto the tower bottoms flow.
· Surge Volume Control (SVC) on the overhead condenser accumulator cascading onto the overhead reflux
· Tower overhead pressure control cascading onto overhead vapor flow.
· Overhead reflux-to-feed ratio.
· Middle reflux-to-feed ratio.
· Hexane Draw-to-feed ratio.
· Overhead reflux-to-feed ratio.
· Reboiler Duty-to-feed ratio.
The scope of an LNG project is to install MVC controllers on the following units
Fuel Gas Stripper
Deethanizer
Depropanizer
Debutanizer
Depentanizer/Dehexanizer
Propane Refrigeration Cycle
Fuel Gas Stripper / Deethanizer MVC-based Controls
A single stand-alone MPC controller is recommended for the fuel gas stripper and then euthanized. Objectives of the controller include:
· Stabilizing the fuel gas stripper operation.
· Stabilizing the deethanizer operation.
· Maintain the C2 specifications in the deethanizer bottoms propane product.
· Optimize the tradeoff between reducing the propane losses in the deethanizer overhead and energy consumption. The LP optimizer embedded in the controller will be used to optimize propylene losses. The refrigeration cost generated in the refrigeration calculation modules will be used by the optimizer to optimize propylene recovery.
· Optimize the tradeoff between reducing the ethane losses in the fuel gas and energy consumption. The LP optimizer embedded in the controller will be used to optimize propylene losses. The refrigeration cost generated in the refrigeration calculation modules will be used by the optimizer to optimize propylene recovery.
· Maximizing the fuel gas stripper throughput limited by operation and equipment constraints.
· Maximizing the deethanizer throughput limited by operation and equipment constraints.
· Operating the fuel gas stripper at optimum pressure without violating tower constraints.
Operating the deethanizer at optimum pressure without violating tower constraint
Depropanizer MVC-based Controls)
A single stand-alone MPC controller is recommended for the depropanizer and the MPPD hydrogenation reactor. Objectives of the controller include:
· Stabilize the tower operation.
· Minimize C4 in the overhead.
· Maximizing unit throughput limited by operation and equipment constraints.
· Operating the tower at optimum pressure without violating tower constraint
Debutanizer MVC-based Controls)
A single stand-alone MPC controller is recommended for the debutanizer Objectives of the controller include:
· Stabilize the tower operation.
· Control the %C5 in the overhead.
· Control %C4/C5 in the button.
· Maximizing unit throughput limited by operation and equipment constraints.
· Optimizing the trade-off between throughput and yield and utility costs. Unit
· Operating the tower at optimum pressure without violating tower constraint
Depentanizer MVC-based Controls)
A single stand-alone MPC controller is recommended for the depropanizer/hexane tower. Objectives of the controller include:
· Stabilize the tower operation.
· Control top pentane product.
· Control middle hexane product.
· Control C7+ bottom product.
· Maximizing unit throughput limited by operation and equipment constraints.
Operating the tower at optimum pressure without violating tower constraint
MPC Controller Implementation Plan
The MPC controller system implementation consists of the following phases:
· Controller Design.
· Plant testing and Model Identification.
· Factory Acceptance Testing
· Controller Commissioning
· Documentation
MPC Controller Design
These preliminary structures will be reviewed with the plant technology and operations personnel.
The objective of the review is to ensure that the recommended control structure accounts for all process constraints. The reviewed controller configuration will be the basis for conducting the plant tests and will be finalized only after the plant test is completed.
Plant Testing and Model Identification
Data collection and identification software provided by ACSC will be used to develop the controller dynamic models. The data collection system should be installed and run during the tests.
A pretest of the regulatory system will be conducted before the response test. This test involves stepping each of the manipulated variables and monitoring the regulatory controller response. This provides information on the settling time of the process and the size of the moves required during the response test
Using ACSC supplied identification package will considerably facilitate the testing procedure and eliminate the need for conducting the tests on a 24-hour per day basis. ACSC will provide the LNG plant with a guideline test procedure outlining the independent variables moves and the state of the process during the test. With the information provided by ACSC, the control room operators should be able to conduct the test without the need for the 24-hour engineer’s coverage of the test.
Factory Acceptance Testing
As the plant testing and dynamic model identification of the controllers are completed ACSC will install its proprietary simulation package to tune the controller. The same simulation package will be used during the factory acceptance test to demonstrate the controller behavior in response to setpoint and disturbance changes with different sets of active constraints.
Controller Commissioning
Once the dynamic model has been verified, the controller is commissioned. The performance of the controller is evaluated in an online environment. The controller is first operated in a predict-only mode where the control calculation can be observed without implementing it. All controllers’ functionality is tested in this mode including bad data handling, manipulated variables control modes, and so on.
If tuning changes are required based on the actual performance, they are made at this point. Tuning of the controller takes place and the operators are shown how to operate the controller displays. The specific objectives of the controller are also explained to the operators.
Formal training of operations staff is provided by ACSC technical personnel one month after the controller has been commissioned. The delay in giving the training session ensures that each of the operators has had direct exposure to the controllers