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By Dr Rajagopalan Srinivasan Department of Chemical & Environmental Engineering, NUS |
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Introduction |
While man has been brewing chemicals in batches for long, the technology for batch process automation is only now coming of age. One key development that promises to bring more intelligent-, advanced control applications to batch plants is the S88 batch automation standards. Batch processing is ubiquitous. It is most prevalent in pharmaceutical, food processing, and specialty chemicals industries. However, even industries such as petroleum refining which are normally associated with continuous processing have batch-like operations peppered all over the plant. Batch processes have historically been operated and controlled manually. Over the last two decades, while automation of continuous processes improved by leaps-and-bounds, batch automation continued to be plagued with unfulfilled needs. A tower of Babel state-of-affairs was prevalent in batch control for a long time. Different companies and even different groups within the same company used different conceptual models and jargon when describing batch processes. So a term such as "recipe" could mean different things to different people. A lack of a widely accepted batch control model made talking about operating procedures difficult. A similar dearth of communication protocols made integration of devices from different vendors nearly impossible. These problems persisted for a long time and the recent S88 standard promises to be the elixir for batch automation. |
S88 |
S88 is the international standard for batch control developed by the International Society for Measurement and Control (ISA, previously Instruments Society of America). S88 defines terminology and models for batch control and thus characterize and define the batch process itself. The standard is intended for people who are involved in designing or operating batch plants, responsible for specifying controls and associated application programs, or involved in the design and marketing of products in the area of batch control. The standard comprises of two parts: ISA-S88.01 Part I: Models and terminology and Part II: Data Structures and Guidelines for Languages. Part I has been published already and Part II is still being developed by the SP88 committee. Part I of S88 defines the common terminology
and models to be used in batch processing. It describes how to partition
product-, production- and control- knowledge. These are represented in the
recipe model, physical & process models, and the control activity
model. These models for batch
automation apply not only to fully automated systems but also to
semi-automated and completely manual ones. A recipe is the minimal collection of
information that defines the manufacturing requirements for a specific
product. Fundamental to the practical application of recipes is the
concept that different parts of an enterprise use information about the
manufacture of a product for different purposes and hence require varying
degrees of specificity. S88 defines four types of recipes. The recipe model as shown in Figure 1
details the four different types, their contents and how they are
related. The physical model or the equipment model describes a hierarchical organization of the physical assets, that is, the process equipment. The model has seven levels – Enterprise, Site, Area, Process Cell, Unit, Equipment Module, and Control Module. A control module is a collection of sensors, actuators, and basic-control-logic programs that act as either a regulating device, a state-oriented device, or a combination of these such as a block-valve controller. Equipment modules are a grouping of both equipment and control modules that can carry out a number of minor processing activities such as dosing or weighing. An equipment module usually contains a prime piece of processing equipment such as a fill-and-weigh gauging system and some auxiliary items. A unit is a collection of equipment modules, control modules and the associated basic-control-logic programs that can carry out one or more (classical chemical engineering) processing activities such as react, crystallize, mix, or grind. Units only handle a single batch at a time. The process cell is at the top of the equipment hierarchy and is the collection of all equipment required for a production of one or more batches. |
Figure 1
Recipe types model defines the different types and relationships
between them
Good practices in batch automation are captured in the control activity model. Several
control functions must be implemented to successfully manage batch
production as shown in Figure 2. These control functions define how equipment in the plant will be controlled. The control activity model is a hierarchical model that deals with the functional activities of a batch System and provides an overall perspective of batch control. The relationships between the various control activities are achieved via information flow between them. a)
Recipe management is made up of the control functions that
create, store, and maintain general, site and master recipes. The overall
output of this activity is a master recipe made available to process
management. b)
Production planning and scheduling is the decision process
associated with producing a batch schedule to be provided to process
management. c)
Production information management involves collecting, storing,
processing and reporting production information. d)
Process management includes the collection of control functions
that manages all batches and resources within a process cell. Control recipes are created from
the master recipe, individual batches initiated and resource conflicts
resolved. e) Unit supervision control activity ties the recipe to equipment control via process control. f)
Process control encompasses procedural and basic sequential
and regulatory control. g)
Personnel and environmental protection control activity
provides safety for personnel and the environment. The physical, recipe and the control activities models work
together to carry out effective batch automation. S88 offers several benefits for batch
process design and operation by: a)
Reducing process commissioning time and the time for a new
product to reach full production levels b)
Allowing rapid integration with business-wide planning-and
scheduling systems and thus reduce inventory c)
Reducing cost of batch process automation and efforts in its
life-cycle engineering d)
Enabling vendors to supply appropriate tools for batch
control e)
Allowing interoperable, best-in-class tools from multiple
vendors to be integrated The use of S88 models in a plant would also open other vistas that are now becoming available. New classes of intelligent software applications that automate operating procedure synthesis, monitoring batch operations, and even safety (HAZOP) analysis have been reported in scientific literature. As these technologies mature and are implemented in plants, batch processing would be ready to face the challenges of another millennium. |
Figure 2
Control Activity Model provides an overall perspective of batch control and the relationships between the various control activities
Further Reading |
1.
ISA, S88.01, Batch Control Part I: Models and Terminology,
1995. |
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