A plant can go to a new production rate within minutes by plant wide feedforward control. If all of the key flows (feed, utility, recycle, and vent flows) are set to the values on the Process Flow Diagram (PFD), the plant can move to a new production rate without waiting on composition, level, pressure, pH, and temperature loops to respond. Feedforward control can enable flexible, sustainable manufacturing.
The new weekly publication Sustainable Plant provides a wealth of details on how to improve the efficiency (reduce raw materials, utilities, and waste), flexibility, maintainability, operability, profitability, and safety, all of which ties to together in terms of compliance and competitiveness. Plant wide feedforward can help achieve all of these objectives by not waiting on feedback correction to realize these goals while meeting a demanding market in terms of production rate and product grade flexibility.
The PFD values may not be a simple ratio of the new to old production rate although this could be a good starting point. Better would be sets of flows captured at different operating rates and ambient conditions (e.g. summer versus winter operation) for the flow feedforwards. Process PID loops would as usual trim through feedback correction the feedforwards. The concept can be expanded to product grade changes and fed-batch operation for faster and more adaptive transitions and batches.
This brings us again to the concept of Live Process Flow Diagrams. What if the PFD used to define the design of the plant was put online to define the current state of the plant?
You may have cascade control and several layers of loops but ultimately what is manipulated is nearly always a flow by means of a control valve, damper, or variable speed drive. Flows are the inputs and outputs of unit operations in the process industry. While Coriolis meters provide the ultimate knowledge in terms of mass flows and compositions via densities, wireless multivariable DP flowmeters with pitot tubes can get you started. Try putting flow measurements on every flow in a unit operation and see what you can learn and can do automatically to move this unit operation to a new production rate by means of feedforward. When you see the benefits, move on down the line. Wireless portable instruments can offer demos of how much faster and smoothly you can move to new rates and grades.
If a plant can respond quickly to changes in raw material supply and market demands, the inventories can be reduced. If a plant can increase rates when utility costs are low (e.g. off peak time rates) and ambient temperatures are low for cooling water, the energy efficiency can be increased.
The advantages of adding flow measurements extend way beyond feedforward. The opportunities for plant knowledge, online process metrics, modeling, and data analytics greatly improve through measurement of the unit operation inputs and outputs, the flows. Advances in flow and level measurements enhance process knowledge, control.
Flexibility in manufacturing and life means being able to “roll with the changes” making the most out of “going with the flow” and using feedback as a reality check. Are you ready to make a break with the past and go with the future?