(22b) Advantages of "Grey Box" Multivariable Predictive Control for Ethylene Plants

After the upgrade of a Lower Olefins steamcracker, an Advanced Process Control (APC) project was initiated to achieve a virtual expansion using software. The project team developed one large integrated Shell Multivariable Optimizing Controller (SMOCPRO) across the whole ethylene plant from furnaces to ethylene and propylene distillation columns. RQEPRO technology is used to provide robust quality estimation. The project increases plant throughput by pushing feed to the furnaces while respecting all process conditions in furnaces and separation sections (operating window). While maximizing production, energy consumption is reduced where possible.

Project phases

In order to phase the rollout of all advanced controller applications, the project was implemented in three phases. In first phase, an independent SMOCPRO application was been developed for every furnace and distillation column. Also, the cracked gas compressor and the refrigeration compressors were included in the APC scope. During phase two, all controllers were merged into one controller model. This phase is also subdivided into a number of steps where the stand-alone controllers were gradually merged into larger controllers and sub-controllers. Phase three, the final phase, is the actual integration with the olefins production controller. After Phase 3 production can be maximized or targets can be entered as set points, set ranges, or limits. Based on these targets and plant conditions, the furnaces feed rate and operating conditions are adjusted by the SMOCPRO controller to maximize production.

New/innovative APC technology

The project leverages many substantial innovations in APC technology incorporated in Shell Global Solutions' APC package (SMOCPRO). SMOCPRO features:

• Advanced modeling capabilities including intermediate variables cascade correction of manipulated variables and explicit options to model the magnitude and dynamics of unmeasured disturbances;

• The concept of subcontrollers, which allows the control designer to break a unit-wide controller down into smaller sections that ease the tuning of the controller as well as reducing the computing time of the unit-wide controller;

• Nonlinear steady state optimization, which allows the control designer to write simple economic functions;

• On-line updateable model gains, allowing the control designer to employ gain scheduling and nonlinear control techniques;

In order to adapt the APC solution to ethylene plant specific needs the custom applications were developed.

Performance monitoring

In order to sustain the application after project completion, it is important to define performance indicators that will monitor performance. Target sheets and performance curves are developed in order to continuously show stakeholders that the system is performing well. MDPRO is Shell's process control monitoring and diagnostics tool, which is used online and offline.

Concluding remarks

This project successfully integrated APC applications, which control a complete ethylene cracker complex.