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Sep
08

Batch vs. Continuous Control and Optimization – Part 1

Feedback control systems play an important role in batch and continuous unit operations. What are the similarities and differences in the process response, tuning, control strategies, and advanced process control opportunities? What can continuous control specialists learn from batch control experts and vice versa? Is it possible there are some fundamental principles unifying our solutions extending the applicability? What makes a process batch or continuous?

Here in part 1 the focus is on the distinguishing features of the processes and products. Part 2 of this series reveals the dynamics and tuning that are seemingly different but can be treated on a common basis. Part 3 gets into the similarities and differences in temperature, pressure, and end point control, and optimization. Part 4 discusses the distinctive composition control strategies for batch and continuous operations.

What distinguishes a batch from a continuous process simply comes down to whether the discharge flow is continuous or cycled.  Whether the discharge flow is positive or zero during processing determines the differences in dynamics, control, and optimization. A fundamental understanding of the impact of discharge flow can lead to insights on the opportunities for continuous and batch processes.

In all of the batch processes I have encountered the cycled discharge flow is a liquid or solids or mixture of both (slurry).  There was often a continuous overhead gas flow to a vent,  recovery, or distillate system for pressure control. I am sure there are totally gas batch operations where the gas discharge flow is cycled. I just haven’t worked on any.

Fed-batch processes where the feeds are flowing simultaneously under PID control rather than sequentially charged are termed semi-batch or semi-continuous. The better term is fed-batch since the discharge flow is zero while feeding.

The shut down and restart of continuous processes has lead to the statement; “a continuous operation is just a long batch.” While true in terms of there being a time when the process may be idle, the view plays into the misunderstanding of the fundamental differences between batch and continuous. In reality, the startup of a continuous process is a batch operation until the equipment is filled and reaches correct operating conditions and the discharge flow can begin. The expertise in batch sequencing could be usefully employed to  improve the safety, efficiency, and speed of the startup by automation. We will find that the PID  strategies developed for temperature, pressure, and end point control are applicable to both batch and continuous reactors. We can lean a lot from the different disciplines. I was fortunate enough to work for a company with extensive batch and continuous processes.  

Continuous operations are capable of producing higher production rates with smaller capital investments. However, the continuous discharge flow from a well-mixed vessel by definition means that some of the feed entering the vessel is exiting before being fully processed. The amount of time a given volume of contents or feed spends in the vessel varies considerably with the most frequent time being the residence time defined as the vessel volume divided by the discharge flow rate. Also there can be a build-up of by-products from undesirable side effects since the vessel is never completely emptied except perhaps on shutdown. Incompletely processed raw materials are returned and contaminants are purged in continuous operations. The equipment and control systems for recovery, recycle, and purge add considerable complexity.

In inline equipment with negligible back mixing, such as extruders, blenders, kilns, static mixers, plug flow reactors, sheets, and fluidized bed reactors, any given volume of feed spends the same amount of time in the equipment. The time spent in the equipment is the time it takes to go from the entrance to the exit, which is the transportation delay. These processes are deadtime dominant with respect to the manipulation of feeds. Since there is no back mixing, the residence time is a deadtime (transportation delay) rather than a process time constant. Some think of these as semi-batch operations because none of the feed exits until it transports the length of the equipment. There is a profile of temperature, viscosity, and composition with respect to length just as there is a profile of these process variables with respect to time in a batch.

Batch operations are capable of mimicking laboratory experiments reducing the process development and design complexity. Commercialization is much faster with batch unless the process is extremely simple (e.g. blending).

Candidates for continuous operations are products with a high volume requirement, fast processing, minimal side effects, expired patent protection, and extensive history of process R&D. Oil, gas, chemical intermediates, petrochemicals, and commodity chemicals use continuous operations. Extensive integration of unit operations for energy recovery and recycle of materials offers complex opportunities for optimization.

Candidates for batch operations are products with high profit margins, low volume requirements, slow processing times, low side effects, and minimal process R&D time. Specialty chemicals and especially biopharmaceuticals are produced by batch processes. For new biopharmaceuticals with the patent expiration clock ticking and extraordinarily high prices (e.g. > $1000 per gram), the time to market supersede any consideration of process efficiency. Additionally, these processes are extremely slow requiring days to weeks for the cells to produce product. Also, the buildup of dead and mutated cells, toxins, inhibitors, viruses, and bacteria must be prevented by emptying, cleaning, and sterilizing the equipment after each batch. All of these characteristics translate to a predominance of batch bioreactors. Product purification may be continuous.

Some mature biopharmaceuticals are produced by “perfusion” processes operating in the continuous mode for months at a time with extensive recycle. Periodically, these processes are shutdown for decontamination. The well known and robust nature of bacteria used and the high volume requirement for waste treatment have lead to the large scale use of continuous activated sludge processes.

Thank goodness my processing of food and beverage is a batch rather than a continuous operation.