Batch process manufacturers have long understood that applications which require both sequential and continuous control have been a challenge. A typical example is a centrifuge application commonly found in Biotech and Pharmaceutical manufacturing processes. A centrifuge separates solid and liquid material by spinning a sieve-like device at high rate of speed, and recovering the liquid, solid or both materials.
I caught up with Brian Crandall, a Life Sciences industry project engineer. He said that proper control was critical since centrifuges are quite expensive, and sensitive to a variety of failure conditions. These conditions need to be addressed within seconds to prevent equipment damage and possible injury to operations staff.
Brian summed up the control challenge as the centrifuge having various operational states. Moving between these states is best done using sequential operations. However, monitoring of the failure conditions, which change in severity and action depending upon the operational state, must be done in parallel with the sequencing.
If a failure condition occurs, the current sequence has to be stopped, and the specific failure sequence started within a minimum timeframe. The S88 Batch Model defines a sequential state driven approach, but it does not fit the requirements of this application. The big issue is the failure monitor does not offer continuous monitoring required for quick reaction to the failure condition.
Using a DeltaV system for this particular Biotech application, Brian designed, tested and implemented a modified S88 state model that had the ability to stop a sequence without waiting for a transition thus meeting the high-speed timing requirements of the equipment. Multiple sequences would be required for the main sequence, shutdown, and E-Stop to allow stopping one sequence and starting another at the same time. Also the code design needed to modular to fit the rest of the S88 modular design philosophy. Also, this design placed control at correct level in S88 batch model, at the equipment module level.
Some failure conditions the design addressed included: high vibration, VSD fault, low seal water pressure, and low instrument air pressure. Depending on the state of operation, the failure conditions required different actions per a failure condition matrix.
Other industries have applications requiring this mix of continuous and sequential control. Some examples include a refiner in the pulp and paper industry, extruders in the specialty chemicals industry, and other state-driven processes or equipment.

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