Ask Greg McMillan

Greg's Response:

Startups and product or raw material mix transitions performance is particularly nonrepeatable because of manual actions, operator skill limitations and variations, complex dynamics due to a nonequilibrium, and large dead times from low flow rates. Consequently, startups and transitions are vulnerable to Safety Instrumented System activation and excessive losses in process efficiency and capacity. 

Digital Twin dynamic first principle models along with operator training sessions and interviews can find the best operator actions. Implementation of these actions per ANSI/ISA-106.00.01-2023 “Procedure Automation for Continuous Processes” Standard can be continuously improved by prototyping and testing in the Digital Twin leading to much faster, safer, and more productive startups and transitions. The automation often simply consists of putting key PID controllers in remote output or output tracking modes and setting and holding optimum PID outputs making a bumpless transition to automatic control at the best point in the startup or transition. Level and pressure controls are generally commissioned early in the startup or transition to help enforce material balances. When flows are greater than the low flow rangeability limit of flow measurements, flow ratio control is used. Once process and stream conditions are stabilized, composition, pH, and temperature controllers are commissioned to correct flow ratios. 

The Digital Twin can help find and test the best flow controller location in recycle stream path to prevent an unstable process response due the positive feedback of recycle stream composition changes. The Digital Twin can also help adapt the procedure automation to deal with changes in stream compositions and PID tuning requirements.