Continuous Processes Need Redundancy
An automation engineer at a pharmaceutical firm relates his experiences with converting from batch to continuous in a tablet making operation.
"The biggest improvements were to product yields because we weren't discarding material during start-ups and batch ends,"relates the engineer. "For continuous runs, I could set PID parameters to maintain tighter control responses, and the final product became more consistent because of fewer interruptions."
The plant identified mission critical devices and networks and then developed redundant controls. The plant also needed to take devices and instruments offline for calibration checks and maintenance functions without taking down production.
Three mission critical areas were identified: solution application, air flow and temperature. For temperature, three redundant temperature sensors were used. The control system switched from one sensor to another for maintenance.
The three temperature probes performed constant calibration checks. If a primary probe began to drift while the other two remain constant, then the controls switched from the primary to one of the backups. The controls were designed to alternate among devices at startup to ensure that the backups wouldn't fail due to lack of use.
The process also used a redundant solution skid that contained a pump along with automatic inlet and outlet diverter valves. Based on flow and solution pipe pressure, the system would automatically switch to the backup solution skid and alert maintenance of a solution skid problem.
The controls were set up so that the control variable output to the primary pump would be redirected to the backup pump. After a few seconds to allow the pump to reach speed, the diverter valves would slowly operate to accurately control solution flow.
Once the diverter valves were completely cycled, the loop control would be switched to automatic and the primary pump would be shutdown. If the solution deviated from parameters during this process, the controls would open a dump flapper to dump tablets into a reject bin until the process was brought back in control.
The air flow was controlled the same way as the solution with redundant supply and exhaust fans, motors and drives. If any of the devices failed, the control system would divert immediately and dump accordingly.
The plant had to develop strategies to isolate areas of the process to keep an upstream issue from affecting downstream processes. The controls also had to sequentially bring areas of the process up during start-up, and to sequentially stop areas of the process at the end of a production run.
"Continuous processes work great if you're producing a bulk material. We only have one continuous process, and it was very difficult to switch mindsets from batch. The entire continuous process included other upstream devices, such as weigh feeders and tablet presses that could be isolated with minimal impact to the process. But the continuous coating process is something that couldn't be stopped once the tablets enter the chamber," notes the engineer.
Continuous Processes Safer
A North America-based global manufacturer of aerospace carbon-fiber composites historically produced resins via batching. The batch process was easy to understand and control because of the isolated process cells, but batching forced the manufacturer to produce and store large quantities of volatile resins. Equipment needed to be cleaned between batches, and the plant needed to dispose of the residual material.
With continuous operation, resin production would be directly slaved to and immediately consumed by downstream operations, eliminating the requirements for intermediate storage and reducing cleaning requirements.
After extensive material testing at multiple blender suppliers, a continuous processor from Readco Kurimoto (www.readco.com) was selected (see Figure 2). "Readco's twin-screw co-rotating design has movable and interchangeable mixing elements that can be quickly rearranged to optimize production of different resins,” says Delmar Schmidt, an applications engineer with system integrator Melfi Technologies (www.melfitechnologies.com) in Ontario, Canada. Melfi designed, installed and commissioned the new continuous system.
"Factory test results showed improved consistency of the mixed resin. Continuous operation minimized in-process resin, and also reduced safety concerns because of the closed nature of the processor," adds Schmidt.
"System cleanability was enhanced due to mixing element close clearances and a self-wiping design. The clean-up between product changeovers simply consisted of the addition of a few gallons of solvent or cleaning agent circulating within the blender. Quite often the solvent could be saved and reused, reducing disposal costs," he notes.
The old batching process was a series of disconnected steps that each functioned as an island of automation with minimal control system integration. When the batch process reached a logical stop point, it was easy to measure, check and approve before pushing to the next step.
But the continuous process had to operate in a steady-state condition that required all process equipment to function as a single system. Specific start-up and shutdown sequences were required to ensure the processor wouldn't overload with powders, and the processor had to be purged of toxic or flammable ingredients when not in use.
Most of the suppliers of the liquid and powder process equipment at the plant provided stand-alone proprietary controllers, greatly increasing the difficultly of control system integration. So the manufacturer asked Melfi to devise a solution that would allow the entire continuous process to operate as one closely integrated entity.
Melfi chose to eliminate all of the proprietary controllers and replace them with a Rockwell Automation (www.rockwellautomation.com) solution consisting of a Process Automation Controller (PAC), PowerFlex EtherNet/IP motor drives and a Factory Talk SE touch screen HMI.
Perhaps the main challenge was controlling the continuous loss-in-weight feed subsystem. For control of this subsystem, Melfi linked the PAC to Hardy Instruments (www.hardyinstruments.com) weight transmitters via a dedicated EtherNet/IP network. The feeder control algorithms were then performed in the PAC, while the weight transmitter provided a direct local connection to the load cells.
The HMI displayed and controlled the entire process via a central recipe system. "Now that the continuous process has been integrated into a single system, the plant is enjoying the benefit of reduced manpower to manage the resin line. The composites manufacturer is already planning additional continuous process lines for North America and Europe," concludes Schmidt.