Comparison of batch and continuous control in chemical manufacturing

While often discussed as if they are without overlap, batch and continuous operations share many functions. Understanding these similarities as well as the distinctions, can help you better understand your own operations and new developments in controls.

First, while there are plants that run purely batch or continuous processes, many have both. A batch plant might have continuous operations. A continuous plant is likely to employ batch controls for some processes.

Logically, batch control code is continuous logic plus the capability to routinely stop and start. From this perspective, one could argue that batch controls are more complex than those required for a continuous operations.

This perspective is unlikely to impress managers of continuous operations who know an an hour of unplanned downtime can cost tens or hundreds of thousands of dollars. For this reason, controls for continuous operations almost always include more safeguards to keep processes online. Traditionally, continuous plant manufacturing managers have favored distributed control systems.

Still, as the table below demonstrates, the common ground is considerable.

Another traditional distinction fades as well

If the distinctions between process operations categories are blurring, it should be little surprise another is also fading away: The difference in capabilities between DCS and programmable logic controller-based controls systems.

Today's DCS systems now offer many PLC-like functions, if not integrated PLCs themselves. PLC providers have developed control platforms or systems around their logic units. Whatever the vendors might call them, these are now hybrid systems.

For the most part, this capabilities and features race is good for those upgrading or installing controls. While some differences remain, some now argue that nearly all operations can be successfully managed with either type of system. The choice can come down to familiarity and prefer as much as capabilities.

Comparison of batch and continuous operations

Process	Batch	Continuous
Automated raw material charging	Yes	Yes
Benchmarking	Yes	Yes
Mixing	Yes	Some
Reactor inerting	Yes	Yes
Heating and cooling	Yes	Yes
Data historians	Yes	Yes
Agitator and motor control	Yes	Infrequent
Reactor discharge	Yes	Some
Filter press automation	Yes	No
Spray drying	Yes	Yes
Surge tank	Yes	Yes
Scrubbers	Yes	Yes
Ingredient metering	Yes	Yes
Reflux return/recycle stream	Yes	Yes
Gas separation	Yes	Yes
Columns/Strippers/Overheads	Infrequent	Yes
Batch blending	Yes	No
In-line blending	No	Yes
Filtration	Yes	Yes
Tank farm systems	Yes	Yes
Filling	Yes	Some
Skid and subsystem control integration	Yes	Yes
Process waste water treatment	Yes	Yes
Incineration control and heat recovery	Yes	Yes
Clean/wash	Yes	Infrequent
Real time monitoring	Yes	Yes
Sampling integration	Infrequent	Yes
Solvent recovery	Yes	Yes
Pilot plants	Yes	Yes
Compression and condensing	Infrequent	Yes
Weighing systems	Yes	Yes
Centrifuge	Yes	No