The Eli Lilly processing plant in Clinton, Ind., uses a fermentation-based process to produce a number of animal health products. Solvent is recovered from the fermentation medium and reused within the production process, providing environmental benefits as well as raw material cost savings.
Three recovery columns distill the fermentation medium to recover the 1–2% of solvent it contains. Vaporized solvent is condensed to provide a very pure liquid for recycle. Maintaining high efficiency and speed of separation requires precise control of the temperatures, pressures and flows of components along the column.
The separators also are very important to the overall process mass balance. Inefficient operation can lead to energy waste, output-product-purity problems and poor yield.
The solvent recovery process is continuous and runs year round. The flow rate into each column is roughly 300 liters/minute. This amounts to 125 million liters/year per column — which requires extracting around 6 million liters/year of solvent from each column. The uptime of each column was around 80% due to maintenance, upsets and planned downtime.
Inefficiencies or downtime in the solvent recovery columns can bring the primary production process to a stop after reserve capacity is depleted. Lost production costs could run in the hundreds of thousands of dollars. In addition, because the bottoms from each of the three columns goes to the site waste treatment plant, inaccuracies and column upsets result in lost efficiency at that plant.
Drivers for Change
Companies like Lilly now expect a lot more from process control and instrumentation than in the past. We continually are looking to minimize waste, maximize output and optimize efficiency throughout the plant. There's much more focus on the accuracy and reliability of measurements. With the solvent recovery columns we needed to upgrade the instruments to provide greater accuracy and reliability.
The existing differential pressure/orifice flow meters measuring feed, overhead and bottom flows of the separator columns were around 25 years old and had reached the end of their useful lives. In fact, we actually had stopped performing time-consuming maintenance on these devices. When first installed they had provided adequate flow measurements that even now would be good enough for inventory management. However, they no longer were giving the accuracy or reliability required for the recovery process.
As a result, much of the operation of the columns became manual. Columns were put in recycle mode due to more frequent process upsets. Lost uptime was estimated to cost an average of $3,000 per upset.
Once or twice a month the feed flow measurements were so poor that we experienced excursions outside of the control limits. These excursions could be very costly, requiring isolation of the waste streams and further testing/treatment as well as disruption of other plant processes. We estimated the cost to be as high as $50,000 per upset.
Even without these excursions, we knew the instruments were losing the plant money every day — because the lack of optimized control (as a result of the inaccurate flow measurements) was leading to inefficient energy use, lost solvent and additional waste-treatment costs.
A Cost-Effective Option