1. Nordic Process Control workshop, Telemark, 2009
Development and commissioning of nonlinear MPC controllers in a new polypropylene plant
Nordic Process Control workshop, Telemark, 2009
Staffan Haugwitz, APC specialist
with support from Erik Wilsher, Kai Hofsten
Ketil Strand Andersen, Bjørn Glemmestad
och Øyvind Moen.

2. Products from Borealis plastics
January 11, 2019
Nordic Process Control workshop 2009

3. Overview of polypropylene plant, PP6
Loop reactor
1st Gas phase reactor
Ethylene feed
Hydrogen feed
2nd Gas phase reactor (rubber)
Propylene feed
Catalyst feed
Ethylene, Hydrogen,
Propylene feeds
Ethylene, Hydrogen,
Propylene feeds
To dry-end
part of plant
Prepolymerisation reactor
Contionuous plant with many different product grades.
Strong interactions in addition to strong non-linearities during grade transitions.
Each reactor has its own NMPC controller with 3-4 MV and CV
Polymer
Monomer
January 11, 2019
Nordic Process Control workshop 2009

4. Overview of polypropylene plant, PP6
NMPC
NMPC
NMPC
Loop reactor
To dry-end
part of plant
1st Gas phase reactor
2nd Gas phase reactor
Each reactor has its own NMPC controller with 3-4 MV and CV
Focus on cruise control (control at normal operation and some grade transitions)
Trade-off between fully centralized controller and three individual NMPC controllers
computational complexity, ease of implementation, tuning and maintenence,
January 11, 2019
Nordic Process Control workshop 2009

5. LOOP
GPR1
GPR2
January 11, 2019
Nordic Process Control workshop 2009

6. OnSpot: Controller structure
Plant incl. DCS and
“calculated
measurements”
Estimator
Controller algorithm
Model + _
States and parameters
Measured disturbances
Manipulated variables (MV)
Measurements
Estimated measurements
Controlled
variables (CV)
Developed in-house
First implementation 1994!
January 11, 2019
Nordic Process Control workshop 2009

7. Models and model structure
Simple physical models
We need accuracy of a wide operating range
Avoid too stiff and too detailed models
Avoid model structures that cannot be identified
Validate the model against process data
Important to have a numerically robust implementation
Process modelling can give extra benefits
Identifying errors in sensors and improved operating strategies
January 11, 2019
Nordic Process Control workshop 2009

8. Estimation and model updates
Simple model update (decentralized) – each measurement updates one state and one model parameter. Problem when the states are strongly coupled.
May be difficult to decide which model parameter to update, to get integral action.
The model updates can compensate for disturbances in the process, for example changing catalyst activity
Updates cannot compensate for fundamental errors in the model structure
Tuning the model update is really important for control performance
January 11, 2019
Nordic Process Control workshop 2009

9. Optimization algorithm
SQP (Sequential Quadratic Programming)
Linearize the plant dynamics along the prediction trajectory
Calculate sensitivities with finite differences
Tuning
choose prediction horizon
choose number of MV-blocks
choose how the MV-blocks are divided along the prediction horizon
choose weight coefficients for MV/CV‘s
choose slopes for setpoint ramps (beta-factor)
choose maximum Δu for each MV
January 11, 2019
Nordic Process Control workshop 2009

10. Important things to get right
Interface/communication between NMPC and DCS, e.g. bumpless transfer, correct units, error handling
Identify weaknesses in the mechanical design, e.g. too small/large valves, errors in GC-analyzers, unmixed catalyst tanks. Important to be on site!
Training of the operators, not too early, not too late. Build up local MPC competence.
User-friendly implementation and interface. Activate and deactive prodedures as simple as possible.
Get suitable alarm limits, not too often, but not missing the important events.
January 11, 2019
Nordic Process Control workshop 2009

11. Decr. H2-feed, an indirect disturbance
January 11, 2019
Nordic Process Control workshop 2009

12. 36 hours plot with MPC (loop reactor)
increase production -> increased catalyst feed -> increase propylene feed to maintain density
constant production, but unmeasured disturbance (catalyst activity) forces APC to slowly increase catalyst feed to maintain constant production rate
setpoint increase in production rate -> catalyst increase again
6th column, H2 feed is decreased (measured disturbance), which quickly affects the production rate, no chance to counteract quickly enough with catalyst feed
Setpoint change in density, leads to higher production rate which also needs to be compensated
January 11, 2019
Nordic Process Control workshop 2009

13. 36 hours plot with MPC (loop reactor)
increase production -> increased catalyst feed -> increase propylene feed to maintain density
constant production, but unmeasured disturbance (catalyst activity) forces APC to slowly increase catalyst feed to maintain constant production rate
setpoint increase in production rate -> catalyst increase again
6th column, H2 feed is decreased (measured disturbance), which quickly affects the production rate, no chance to counteract quickly enough with catalyst feed
Setpoint change in density, leads to higher production rate which also needs to be compensated
January 11, 2019
Nordic Process Control workshop 2009

14. Closed loop control of Gas Phase Reactor (2 days)
January 11, 2019
Nordic Process Control workshop 2009

15. manual control for one hour
January 11, 2019
Nordic Process Control workshop 2009

16. Summary
In-house nonlinear MPC used to control each reactor (3-4 MV/CV), MPC now used over 90% of the time
Physical nonlinear model allows a wide operating range for control
Model development is the single most important component in each project. Models are reused from previous projects.
Very important with:
Operator training and acceptance
Communication between MPC and DCS
Local APC competence to ensure necessary maintenance
Next project: NMPC for 400 M€ high-pressure PE plant in Sweden
Borealis is also building a cracker + three more PP/PE plants in Abu Dhabi to be completed 2010, next expansion planned for 2013 with even larger cracker + five more PP/PE/LDPE plants.
January 11, 2019
Nordic Process Control workshop 2009