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Learn about optimizing chemical plant operations for maximum throughput through bottleneck control. Understand different operational modes, throughput manipulation, maximizing profit, and the role of Throughput Manipulator (TPM) using network theory for achieving efficiency.
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Throughput maximization by improved bottleneck control Elvira Marie B. Aske*&, Sigurd Skogestad* and Stig Strand& *Department of Chemical Engineering, Norwegian University of Science and Technology, Trondheim, Norway &Statoil R&D, Process Control, Trondheim, Norway
Outline • Modes of optimal operation • Maximum throughput • Throughput manipulator (TPM) • Max-flow min-cut theorem • Realizing maximum throughput • Single-loop • MPC • Back off • Conclusions
Depending on marked conditions: Two main modes of optimal operation Mode 1. Given throughput (“nominal case”) Given feed or product rate “Maximize efficiency”: Unconstrained optimum (“trade-off”) Mode 2. Max/Optimum throughput Throughput is a degree of freedom + good product prices 2a)Maximum throughput Increase throughput until constraints give infeasible operation Constrained optimum - identify active constraints (bottleneck!) 2b) Optimized throughput Increase throughput until further increase is uneconomical Unconstrained optimum
Mode 2a: Maximum throughput • Typical profit function: • Feed flows are set in proportion to F and assume constant efficiencies: • Leads to: Maximize profit → Maximize throughput F
Throughput manipulator (TPM) Buckley (1964). Techniques of Process Control Price, Lyman and Georgakis (1994). Throughput manipulation in plantwide control structures. Ind. Eng. Chem. Res. 33, 1197–1207.
From network theory: Max-flow min-cut theorem Maximum flow through the network is equal to the capacity of the minimal cut (Ford and Fulkerson, 1962)
Bottleneck • Maximum throughput achieved by maximizing the flow through the bottleneck • If the flow for some time is not at its maximum through the bottleneck, then this loss can never be recovered Maximum throughput requires tight control of the bottleneck unit
Rules for achieving maximum throughput • Maximize flow F through bottleneck at all times • Use TPM for control of bottleneck unit • Locate TPM to achieve tight control at bottleneck • Back off: usually needed to ensure feasibility dynamically Fmax F Fset point Back off Time
Realize maximum throughput Best result (minimize back-off) if TPM permanently is moved to bottleneck unit Max = bottleneck Skogestad (2004) Control structure design for complete chemical plants Comp. Chem. Eng 28 p.219-234
Realize maximum throughput in more complex cases • Bottleneck moves • Multiple feeds and crossovers Proposed solution: Coordinator MPC* • Estimate of remaining capacity in each unit is obtained from local MPCs • Coordinator MPC manipulate TPMs (+ crossovers) to maximize flow through bottlenecks *Aske et al. (2007) Coordinator MPC for maximizing plant throughput Submitted to Comp. Chem. Eng
Coordinator MPC - maximize throughput (CV with high, unreachable set point with lower priority) - TPMs as MVs - keep columns within their capacity (CV constraints) - disturbances moves the bottlenecks CV CV CV CV MV MV MV CV MV CV CV CV MV CV MV CV
Back off = loss (in throughput) • Back off can be reduced by • Improved control (to some extent) • Limited by network dynamics from TPM to bottleneck • Obtain TPM closer to bottleneck • Move TPM (Change in base control) • Add buffer tanks to get dynamic TPMs (Design change) or use existing buffer volumes • Estimate back off to find economic incentive: • Worst case amplification:
Bottleneck Bottleneck Example – estimation of back off • Compare TPM at feed and at bottleneck • Feedback controller K tuned by Skogestad’s tuning rules, τc=3θeff • Disturbance rejection as function of frequency
Back off as a function of frequency • Peak unavoidable • Effect of disturbancesreduced
Conclusions • Tighter bottleneck control can reduce back off • TPM should be used for control of the bottleneck unit to obtain maximum flow • Bottleneck fixed →single-loop control sufficient • Bottleneck moves → multivariable control • Consider moving/adding TPM if back off is large