The best of the best, Part 1

Columnists Greg McMillan and Stan Weiner, PE, bring their wits and more than 70 years of process control experience to bear on your questions, comments and problems in this month’s installment of Control Talk.

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McMillan & WeinerBy Greg McMillan and Stan Weiner, PE

Stan: We are going to have a 6-part series to capture the some of the expert expertise of people we have known while our minds are still sharp or at least coherent. We start out with ISA Fellow and Control Hall of Fame inductee Terry Tolliver.

Greg: What are the two applications you worked on with biggest benefits?

Terry: Long-term operational involvement with two of our chemical intermediates processes provided several million dollars per year in capacity, yield and energy benefits. The operational improvements were implemented and demonstrated by a team approach with key plant contacts.

The first of these processes involved solvent recovery and make-up solvent manufacturing. The solvent recovery area used triple-effect distillation to remove water overhead from the solvent and then used another distillation column on a slip stream to remove a high-boiling impurity as a bottom-purge stream.

An opportunity to produce a higher value product required the solvent area to provide additional water and high-boiling impurity removal. My involvement began with the column taking the slip stream, where its feed rate was limited to 20 kpph by the reboiler heat duty and its bottom purge of the impurity was only 20 pph. Furthermore, its reboiler fouled severely and required downtime for cleaning every two months.

The circulation pump was cavitating, and an axial nozzle on the reboiler was allowing the circulating flow to bypass most of the exchanger tubes. Increasing the diameter of the nozzle and pipe on the suction side of the pump while operating at a higher sump level, provided enough NPSH to avoid pump cavitation and achieve design circulation rate. A baffle was installed behind the axial nozzle on the head of the reboiler to provide proper distribution of the circulating flow to all the tubes in the bundle. The reboiler now operated with less fouling and could be cleaned during regularly scheduled department downtime.

These design changes more than doubled the reboiler heat duty and allowed better control of the sump level by manipulating reboiler steam. We also implemented feed-forward control of the sump level and ratio control of bottoms-purge-to-feed rate. The column capacity was thus increased to 40 kpph feed. The improved control allowed the bottoms to be more concentrated, which increased the purge rate to 50 pph.

A second phase of de-bottlenecking installed a feed preheater and structured packing in the column above the feed.  The structured packing not only provided additional stages of separation and capacity, but also allowed a lower operating pressure and pressure drop, which enhanced the relative volatility of the high-boiling impurity. Feed-forward control was applied to the feed preheat temperature, column pressure and reflux controller set points to maximize throughput at optimum separation. Less variability due to improved control allowed the impurity concentration in the solvent to be maintained closer to its upper limit. With all these changes, the column demonstrated an 80 kpph feed rate and 200 pph impurity removal capability (a ten-fold increase from where we began).

Solvent losses were reduced in the triple effect distillation area by feed-forward control with closed-loop, analyzer-based composition control. The solvent manufacturing process was also de-bottlenecked, leading to excess capacity, allowing external sales of the solvent.

The second process was much more complex, involving both organic and aqueous recycles to a continuous reactor area with numerous parallel reactors. Following a project which added several more reactors, the recycle equipment was pushed to its limits. The process was unstable in that reactor flows and temperatures were continuously interacting, and numerous trip-outs due to low flow rates, high temperatures or separator interface levels kept the entire system from lining out.

My first efforts were directed at providing an override level control strategy for the separators. The new strategy specified a desired flow rate and used feed-forward control to set both inlet and outlet flows. If either flow dropped off because of a plugged filter, the opposite flow was reduced accordingly in order to maintain the interface level. Next, reactor temperature control strategies were modified to minimize interactions, keeping the reactor flow loops from having low flow trip outs. Stabilizing the system not only improved on-stream time necessary to demonstrate capacity, but also enabled optimization of the process.

Temperature control was improved by using variable-speed fans on the cooling towers to allow operation closer to reactor-temperature constraints. Operation at higher reactor temperatures saved considerable energy.

On the aqueous recycle, a critical composition was controlled by the water-purge rate. Control strategies for parallel distillation columns were modified such that flow control of their water distillate could receive remote set points from a model predictive controller. An online analysis of the aqueous composition was made possible by complex sequencing of a batch analytical procedure and inferential measurements, which were corrected by periodic feedback from lab analysis. Significant energy and yield savings resulted from the improved composition control.

Greg: How were benefits reported to help justify additional process control improvements?

Terry: All the benefits for both these applications were attributed to the process control improvement (PCI) team, although clearly in the first application, the second phase addition of the preheater and structured packing involved the installation of new process equipment. However, early in our company-wide PCI efforts, process control improvement was redefined as any idea originated by the PCI team in review of the operation of existing units. Tracking of benefits became an important factor in keeping management commitment to pre-fund the ongoing studies.

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