A shift operator spent half his time adjusting makeup water to the towers via 17 valves," observes Dan Curry, senior process control engineer with Optimation. "Chemicals for water treatment were also added periodically by operators. The manual/batch operation of water and chemical addition resulted in large quantities of wasted water and water treatment chemicals, as well as inconsistent water temperature and chemical concentration. The cooling towers and associated water headers were segregated into five groups, and a level transmitter and control valve were used to automate water makeup and chemical additions."
He adds that "Benefits included reduced operator involvement, reduced energy consumption and reduction of heat exchanger and piping fouling due to lower chemical concentrations. Fewer chemicals were sent to the sewer and less water was sent to drains. Finally, the new system exhibited improved stability of chilled water supply temperature and lower demand of water from the central plant."
The benefits of switching from batch to continuous are substantial, but these benefits can't be realized by overcoming some significant automation challenges.
A Higher Level of Control
Simply put, continuous systems required a much higher level of control than batch processes. "With continuous operation, the control system needs to be able to correct more quickly, reliably and robustly to changes from steady-state," notes Wikstrom.
"With batch systems, there's usually an opportunity to pause the process and make corrections if things go awry. Not so with continuous processes. Operators no longer have the luxury of just pausing equipment, so they have to develop isolation strategies to keep an upstream issue from affecting the downstream processes. (See "Continuous Processes Need Redundancy," at the end of this article for more details on this batch-to-continuous conversion.)
For the aforementioned brewery fermentation processes, loop control of four fermenter tank levels was a particular challenge. "In a typical process, a loop response time might range from a few seconds to a few hours, but this project often had loop cycles of 24 hours or more. Inventive advanced process control techniques were developed to deal with these significant time lags," says Coker. "These techniques consisted of using much smaller gains, and then adjusting the bias for the PID loop based on the differences in the control variable (CV) output to the valve for the upstream and downstream fermenter level controls. Since the flow cascades through the four fermenters to the beer well, the flow out should match the flow in. Flow metering was not available between fermenters, so the differences in the CVs were used to approximate flow and the loop was biased based on that difference."
More Precise Measurement
Batch processes can be paused when things go wrong, but continuous processes often cannot. This means that instrumentation needs to be more reliable, often through the use of redundant transmitters. (See "Continuous Processes Need Redundancy," at the end of this article for more information on how redundancy increases reliability.
Most batch processes use off-line measurements made in a lab to control quality and production. With continuous processes, these measurements now need to be moved on-line. This presents numerous instrumentation challenges, up to and including the use of soft sensors that infer process parameters when no direct measurements are possible.
"A soft sensor is a mathematical model based on first principles and empirical data that can approximate measurements based on various process parameters," explains Ric Snyder, senior product manager at Pavilion Technologies (www.pavtech.com), a Rockwell Automation company.
Hron of Kraft Foods details other measurement problems that can arise with continuous control. "There can be problems when going from a single scale in a batching system to multiple flowmeters in a continuous system. Often, extra calibration work is required along with more data monitoring. Most seriously, continuous processes may need sensors that just don't exist."
Not all processes can be converted from batch to continuous, but for those that can the benefits are substantial. The automation challenges involved with making the switch are significant, but can be overcome. The batch manufacturers that make the switch for all or part of their processes will forge ahead, while those that remain mired in the batch world will fall behind.
Dan Hebert is Control's senior technical editor.
Reactor Goes Continuous
In 2000, Bayer MaterialScience started up the world's first commercial continuous process for the production of polyether polyol at its Channelview, Texas, facility. The process had previously used a batch reactor, one of two reactors on-site.
"Continuous processing is inherently more efficient than batch processing," observes William D. Wray, PE, engineering consultant with Bayer. "However, in many processes, including ours, you can't simply decide you want to manufacture continuously―the technology has to allow it. For us, the switch was made possible by a new catalyst with kinetic properties that allowed continuous processing,' continues Wray.