Batch to Continuous

This article was printed in CONTROL's September 2009 edition.

By Dan Hebert, PE

If your plant has batch processes, significant production improvements can be realized by converting all or part of your processes to continuous operation. These improvements are summarized in Table 1 and detailed in the next section. They include but are not limited to increased throughput, better quality and less energy use.

But realizing these improvements can be quite challenging because converting processes from batch to continuous is complex. Some of the main challenges are listed in Table 2, and many of these issues revolve around the need for more precise control and measurement.

Fortunately, others have blazed a path before you and are happy to relate their experiences. Read on to find out how your peers have successfully converted batch to continuous, reaping the benefits of improved operations.

Why Upgrade from Batch to Continuous?

You know that upgrading all or part of your processes from batch to continuous will be challenging, so why do it? "Continuous operations yield more consistent product quality, more predictable behavior and a better opportunity to maintain and perfect a steady-state process," says Carl Wikstrom, Ph.D., PE, director of research and development at Rineco Chemical Industries in Benton, Ark.

Another end user agrees with Wikstrom and gives some specifics. "Our continuous polyether polyol reactor is about one-third of the size a comparable batch reactor, yet it can produce 1.5 times as much product," relates William D. Wray, PE, engineering consultant with Bayer MaterialScience in Channelview, Texas. "The continuous reactor also features increased energy efficiency, as there is less time and energy spent heating and cooling reactor contents as compared to a batch reactor. Pound for pound, our continuous process uses about 25% of the energy used for the batch process." (See "Reactor Goes Continuous," at the end of this article for more details about Bayer's process.)

A reason cited by an automation engineer at a pharmaceutical plant is that in a continuous process, manufacturers cam increase product yield because they don't have to discard material during start-ups and batch ends. Mark Hron, a cheese and dairy electrical engineer with Kraft Foods, notes the following benefits of converting from batch to continuous. "You can slow down or speed up production to match packing line speeds. Filling and emptying time lags can be reduced. Less operator input and interaction is needed, so labor is saved."

Integrators on Board

Bachelor Controls (www.bachelorcontrols.com) is a systems integrator headquartered in Sabetha, Kan. Bachelor worked with a brewer to convert a fermentation process from batch to continuous (See Figure 1). "The new process has resulted in much more consistent alcohol production," says Marvin Coker, senior project engineer with Bachelor.

"The brewer has seen reduced fouling in the beer still. Clean-in-place costs have been reduced dramatically, as between-batch cleanings have been completely eliminated. There's also been a reduction in raw material costs as the client has not had to pitch yeast into the process since the project went online," adds Coker.

Another integrator details benefits gained by switching from batch to continuous. "Continuous production minimizes intermediate storage of in-process materials that often have relatively low shelf life," notes Delmar Schmidt, an applications engineer with system integrator Melfi Technologies (www.melfitechnologies.com) in Ontario, Canada.

"Significant safety improvements can also be realized if these in-process materials are toxic or explosive," adds Schmidt. (See "Continuous Processes Safer," for more details.)

System integrator Optimation Technology (www.optimation.us) is headquartered in Rush, N.Y. It converted a 90,000 gpm, 17-cell industrial cooling tower water makeup system for a New York-based chemical company from a mostly manual batch operation to highly automated continuous control.

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.)