The Xc still recovers acetone from the effluent of an upstream column for reuse (Figure 4). The recovered acetone must meet a specification of less than 3 wt-% water. During 2003, 615 batches of acetone were processed, 18 of which failed the water specification, leading to significant delays pending the investigation and documentation of each failed batch.
The team used SPC to analyze data from 140 batches, which showed that the acetone had an average water content of 2.3 wt-%, a Cp of 1.24 and Cpk of 0.54. A low Cp indicates a high degree of spread in the data (a Cp of 2 or greater corresponds to Six Sigma performance), whereas the higher the Cpk, the closer the data are to the target. Hence, the data from these batches indicated a lack of control and significant room for improvement.
The first step toward improving column operation was to reconfigure the control scheme and improve the tuning so it could run in automatic mode. Despite the fact that six batches exceeded the 3 wt-% water specification, data from 125 batches showed a 35% reduction in average water content to 1.5 wt-%. Although the average water content went down (Cpk = 0.86), the spread in the data increased (Cp = 0.88).
Other columns onsite were controlled using MPC, so the team decided to implement it on the Xc still. MPC is an add-on module available from Emerson for the DeltaV distributed control system (DCS). The module enables concurrent control of multiple process constraints, rather than managing them as individual loops or variables. One block can monitor up to four different variables and anticipate the expected behavior, thereby applying several corrective outputs as necessary to maintain the optimal column performance. Feito says they use one control block to monitor four inputs and two outputs on the acetone column.
Once MPC was employed, the average water content of 31 consecutive batches was reduced to 1.2 wt-% -- none of them failed – and SPC showed a Cp of 8.98 and Cpk of 7.39. Santiago says there have been no failures since MPC was implemented -- about 600 consecutive batches to-date have been within specification.
Maldonado says that when batches failed the capacity to reprocess the offspec acetone was limited, which might necessitate discarding material and mading up with fresh acetone.
Our judges were impressed by the Baxter team’s innovative use of MPC. “This technology is used widely in the petrochemical industry, but not that much in pharmaceuticals,” Paterson says. Frank adds, “This is a good example of what can be achieved through implementation of improved process control schemes and operating discipline.”
Another recognizes the value of making improvements without a hefty capital investment. “Most batch chemical manufacturing facilities are underutilized,” Worstell says. “Employing computer scheduling software at such facilities provides a means for achieving capacity expansions without capital investment. Implementing such software will increase production and reduce costs, thereby increasing the standard of living for the world community.”