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