Achieve Model Operations

Plants can solve a range of problems by leveraging design models

By Rob Hockley and Ron Beck

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Automatic model execution

Person accepts or rejects any operating advice

 

Real-time closed-loop models
As for open-loop models
Interface directly with plant control system and adjust the process automatically
Table 1. Real-time models are more capable but take more effort to make robust.
Off-line process models represent the first step in re-using design models in a more automated or convenient way. Because they serve an individual plant or operating unit, their topology is fixed and the range of operating conditions is well understood. The models are used for specific calculations such as for: 
  • advising on operating set points for individual equipment items;
  • achieving a reconciled plant mass balance;
  • determining product properties;
  • analyzing energy usage;
  • comparing actual versus design performance;
  • responding to changing market conditions;
  • meeting product specifications; and
  • retaining and enhancing process knowledge.

Even though they may connect to real-time data systems, off-line models aren’t fully automated; a person normally initiates runs.

Models produced during the design phase usually require additional work before they can be used as off-line process models. After all, in design the simulation is created and run by an experienced engineer, who understands the constraints of the model and the range of valid conditions. If difficulties such as convergence failure occur, the design engineer knows how to overcome such problems.

For use in operations, the model must be tuned to match plant conditions and the particular calculations being executed. For instance, the plant setup may change from day to day — with different product grades being produced and individual units or controllers switched on/off. The off-line process models must account for these specifics.

In addition, because the simulations only are valid within a limited range of operating conditions this range must be strictly understood and enforced. The models will need to be made robust, so they always converge within the valid operating ranges. Model inputs (both those entered manually and those coming from real-time data systems) must be kept within these ranges; this often is done by running the models through a simpler custom interface, such as one based on Excel instead of their normal “engineering” user interface.

The sidebar provides other practical pointers for moving models from design to operations.

The next steps

If an off-line process model gets regular use in operations, it may be appropriate to convert it to a real-time open-loop model. The model execution then can be automated to occur, say, once per shift, every N minutes or when triggered by a process event. Such open-loop models also may write results back to the plant’s real-time data systems. However, the results of the model are always evaluated by a person, who ultimately accepts or rejects any advice or data.

 

When moving models from design into operations, pay particular attention to the following points:
  • Number of chemical components. Design models may contain more than are needed in a process model. Fewer components will speed up simulations.
  • Is the model topology up to date? Has the plant changed since the design model was developed?
  • What are the valid ranges for the process model? At what throughput?
  • Does the model need to handle different product grades? If so, you may need a range of alternative models.
  • Does the model need to account for different ambient conditions such as different heat losses and different utility temperatures in winter/summer or night/day?
  • Does changing catalyst activity have to be considered?
  • Which model inputs can be fixed, which will be manually entered and which will come from real-time data systems?
  • What are the lower and upper limits for all model inputs?
  • Some equipment models may require changing from design to “rating.” For example, in design the heat exchanger is specified by outlet conditions with no utility stream included. In rating, both sides of the heat exchanger are included and simulated with heat-transfer coefficient and area.
  • Distillation column efficiencies may have to be matched to plant data or equilibrium-stage models converted to mass-transfer-based ones.
  • Can the plant still operate with some equipment switched off? If so, the model will need to account for this.
  • What are the keys results to be calculated by the model?
  • Which equipment items can be deleted from the design model? Which of these are not required for the particular on-line calculation?
  • Must any additional equipment items be included? For example, long pipes, valves and pumps sometimes are left out of design models.
  • How robust is the design model? Can it cope with the different input values in the plant model?

 

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