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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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