Stan: A process control specialist may be expected to walk into a control room and within a few hours develop wondrous solution to a problem whose real cause not apparent. I had to do this in my career repeatedly on startups when time was precious and patience was slim.
Greg: I also had this eye opening experience that got more technically complicated when I moved into Engineering Technology and was tackling problems that plagued production units for years. Process control improvement comes down to putting process knowledge in the control system. How you get this information from the plant documentation, and operations, process engineering, and research is a problem it itself. Documentation is more focused on details to build and operate the plant than how to control the plant. Research reports may stay in the research department. Chemists, operators, and chemical engineers don't generally understand dynamics or the basics of process control. What they can offer is very important in turns of what does and doesn't work and process relationships. I have found Tip #51 Seek Conversations with Knowledgeable People (7/13/2012) post on the ISA Interchange Site to be essential for success. To get another perspective, we asked James Beall, our guide in our recent December and January columns, how he approaches the challenge of intertwined problems that have evaded solution.
James: I have had to tackle a double effect evaporator system with downstream centrifuges and a lab analysis just once a day, over 20 pieces of equipment, multiple sources and destinations, and a recycle stream of unknown composition. I drew a process sketch that combined the automation system components from the Piping and Instrument Diagram (P&ID) and key streams from the Process Flow Diagram (PFD). Every week I asked more questions and used the answers to improve the sketch.
Greg: There is a non-self-regulating effect of recycle streams where impurities and inerts can accumulate. I know a plant whose capacity did not achieve the promises of debottlenecking projects for decades because a long-forgotten impurity was building up. A research report from over 50 years ago buried in the archives provided the revelation that could have saved millions of dollars and decades of frustration and speculation. There must be an analysis of all components in the recycle stream and an intelligent purge of impurities and inerts. Recycle streams can cause your thinking and the control system to go in circles. Feedback control adds self-regulation. A 1993 series of papers by Bjorn Tyrus and Bill Luyben "Dynamics and Control of Recycle Streams" concluded there must be a flow control of the recycle stream somewhere in the recycle system to prevent a "snowballing" effect. Also, the makeup of a reactant recovered from the recycle stream in a distillation column must fed back to the reactor by tight level control in the distillate receiver. A solution often needs to address the short term effects by feedback control and rely on optimization levels to find the best recycle flow. A paper Model Predictive Control for Process Improvement by ISA Mentor Program protégé Flavio Briguente in the OSU Automation Society Newsletter Spring 2013 showed how an innovative application of advanced process control greatly reduced the pH variability in a reactor with a recycle stream.
Stan: James, what do you think will be the key to a solution for your complex recycle system?
James: I will determine basic models of the process responses to better understand process relationships and operation. I did some initial step tests and found that steady state was reached in the composition response within 2 to 3 hours, instead of the 24 hours expected by operations. The temperatures were also lining out in the same time frame. Part of the misunderstanding may be caused by the once per day sample analysis. During the tests we took extra samples. Process engineers are not particularly in tune with dynamics. Operations may have a more realistic view.
Stan: Since operators spend 12 hours whereas process engineers an occasional few hours with the process, I would expect the operators would have a better sense of time. However, people in general have problems with anticipating the effect of dead time. I found this in retirement.