A Case History of the Implementation of an S88-Aware Batch Control System

Joe Hancock, Eutech

As part of a major project to build a new film production plant at ICI Films'(now DuPont Polyester) Dumfries site, a requirement was identified for a batch plant to manufacture a feedstock additive slurry. The chemical process was reasonably well understood and Eutech (Cheshire, UK) was appointed as consultant, initially to propose the most appropriate control system solution and subsequently to deliver the solution. The control system project was initially highly timescale critical, starting in August 1996, with production trials scheduled for January 1997, although external factors delayed completion until April 1997.

Figure 1 is a simplified section of the 110 I/O plant, showing the dispersion vessel unit, the stock tank unit and some of their associated equipment.

To simplify the control system requirements, the process designer offered the following restrictions on essential functionality:

• Only one batch would be in the plant at any one time.
• Small number of recipes, although some scope for the development of experimental recipes was required.

One of the benefits of the S88 standard is the emphasis on separating what the plant equipment is capable of doing from how it is controlled to make a batch of a particular product. This helps to ensure that a flexible batch control system is developed.

Referring to the plant schematic shown in Figure 1, each valve and each motor should be considered to be control modules. The powder A addition system, consisting of its conveyor system and valve, satisfy the definition for an equipment module.

But how should the other control modules be partitioned into equipment modules? The S88 standard appears to offer minimal guidance in this respect, so we used our experience with object-oriented software development, considering S88 equipment module partitioning to be analogous to the identification of object boundaries. A simplistic approach might consider that any control modules that are physically connected ought to form part of the same equipment module. Taking this approach, valves V1 to V6 would form part of the same equipment module. This would compromise the goal of producing an intuititive model of the plant because it would obscure the ability of the dispersion vessel and the stock tank to feed the mixer and the mill concurrently via valve V5. An alternative approach would be to define several equipment modules, but this has the disadvantage of introducing dependencies (or "coupling," to use a software engineering term) between the control of the equipment modules. Our solution was to consider that a control module belongs to an existing equipment module unless it can undertake a process-oriented task independently of the equipment module. This ensures maximum flexibility with minimum coupling.

Although the plant schematic in Figure 1 appears at first sight to be very simple, in practice there exist several real and difficult issues, not least of which was the control module partitioning mentioned above, to be resolved during the S88 analysis.

A poor partitioning could easily result in a solution that:

• is difficult to support and enhance.
• fails to exploit the inherent flexibility of the plant.
• requires the control system developers to assist the process specialists when developing new recipes.

S88 encourages control system flexibility matching the plant equipment flexibility in order to prevent unnecessary operating constraints imposed by the control system alone. This flexibility was utilised

shortly after the plant began beneficial operation:

• A requirement arose to exploit the concurrency of operation available in the plant, violating the initially offered process requirement restricting the plant to serial batches. To increase throughput, a dispersed slurry would be transferred to the stock tank such that the milling could be done from the stock tank while the next batch was being prepared in the dispersion vessel. As a result of adopting S88, new recipes were easily developed to satisfy this requirement, with no modifications necessary to PLC software.
• The initial requirement defined a small number of recipes. But having installed the system, a number of alternative cleaning recipes were designed, by the client, to satisfy different needs. Once again, the flexibility inherent in the S88 analysis, along with the standard configuration facilities offered by the batch control system platform, ensured that the new recipes were rapidly and correctly configured.
• The process expert has edited existing recipes and designed new recipes without reference to control system development personnel.
• As anticipated, the software platform functionality over and above the user's initial requirements provided opportunities for rapid and minimum cost system enhancement. An example of this is the batch logging capability, providing a batch history for post-batch analyses.

Consequently, on balance, it has been shown that the desired benefits arising from the application of S88 and the use of an S88-aware platform have largely been met. Although reservations were expressed regarding the adoption of a new approach and platform software, the project ran to time and budget, and significant learning has been achieved relating to an S88 methodology and its implementation on current platforms. This learning is being employed in subsequent batch process control projects.

However, while it is clearly much too early to have evidence on the long-term supportability of the control system, the current indications are very positive.

Our ongoing experience with batch control applications suggests that, with appropriate interpretation, S88.01 and S88.01-aware control system platforms can offer significant benefits for some types of application but may be marginal for others.

Eutech, an engineering consulting firm located in Cheshire, UK, helps pharmaceutical companies deal with issues of regulatory compliance, manufacturing effectiveness, and safety/health.

For more information: Joe Hancock, Eutech, Brunner House, Winnington, Northwich, Chesire CW8 4RN, UK. Tel: 44-1606-704075. Fax: 44-1606-704733. Email: joe.hancock@eutech.com.