Table 3131 Control Activity Model




Level 1

Level 2

Level 3

Process/product management Production management

Batch management

Unit management Sequential/regulatory/

discrete control Safety interlocking

Production planning, inventory planning, and general recipe management Recipe management, production scheduling, and batch history management Recipe generation and selection, batch execution supervision, unit activities coordination, and log and report generation Unit supervision, allocation management, and unit coordination Device, loop, and equipment module control, predictive control, model-based control, and process interlocking lected and accessed by the batch management activity which converts it to a control recipe. The control recipe is the batch-specific recipe that is ready to run. Figure 3.13.5 shows the recipe hierarchy.

Using process and product knowledge, the recipe management function analyzes a process and determines the basic phases. These basic phases, along with product knowledge from the laboratory chemist, are used to construct the general (corporate-wide) recipe. Plant knowledge (for example, raw material availability) from the plant site engineer is used to transform the general recipe into a site-specific recipe. Equipment knowledge (for example, what vessels and piping are available in the plant) is used to transform this site-specific recipe into a master recipe. This master recipe is used as the basis for a control recipe when a batch is ready to be produced. Figure 3.13.6 summarizes the activities involved during recipe management.

Production Scheduling

Schedules serve as a guide for the production requirements in terms of the availability of equipment, personnel, raw materials, facilities, equipment, and process capacity. The schedule should have many of the following objectives:

FIG. 3.13.4 Procedure model.

• To minimize the processing time

• To minimize the deviation from a master plan

• To optimize the production of the product within quality guidelines

• To minimize energy costs

• To minimize the use of raw materials

The responsibility of the production scheduler is to develop a detailed, time-based plan of the activities necessary to achieve the production targets set by the production plan. The production scheduler must be able to dynamically allocate a new schedule at any time. Reallocating or creating a schedule automatically via some algorithm or manually via user intervention should be feasible.

Schedulers can be implemented in several ways. Linear programs, expert systems, or other multivariable techniques have been used successfully. The scheduler must provide a procedure and method for batch sizing and is the logical place where lot assignments are made. Figure 3.13.7 presents the production scheduling model currently defined by ISA standards committee (Jensen 1994).

FIG. 3.13.5 Recipe model.

FIG. 3.13.4 Procedure model.

FIG. 3.13.5 Recipe model.

FIG. 3.13.6 Recipe activities.

As shown by the model, the production plan is input to the scheduling model. The plan is first transformed to an area plan. Knowledge about the process equipment is required at this time. The area plan is a listing of the end items which are to be produced, how many of each item are to be produced, and when the items are to be produced for the specific plant area. The area plan is a dis-aggregation of the production plan specific to the plant site and directly drives the production schedule. The transformation to an area plan occurs each time the production plan is sent to the production scheduling function.

The area plan, along with information from the site recipe from the recipe management activity, is used to create the master schedule. The master schedule is a list of the recipes in the order that they are to be run. Lot numbers are optionally assigned, the train or line is determined, and the batches are sized. The master schedule is prioritized according to the production constraints found via the site recipe and is passed to the queue manager. The master schedule can be filed away and is a copy of the best schedule for that area.

Batch History Management

Batch history management involves collecting and maintaining integrated, identifiable sets of dissimilar data. Batch tracking is the collection of this data. It is generally event triggered and contains the following related data:

Continuous process data (flow, temperatures, and pressures)

Event data (operator actions, alarms, and notes) Recipe formula data (set points and times) Calculated data (totalization, material usage, and accounting data)

Manual entries with an audit trail (location of change and operator of record) Stage, batch, and lot identification Time and date stamps on all data

The batch end report typically includes a copy of the recipe used to make the batch. Events such as alarms, operator instructions, and equipment status should also be logged. A trend chart can also be retained. Batch management records and collects batch end reports, which are then archived to some other medium. Batch reports are statutory requirements in some applications (e.g., in the pharmaceutical industry). Figure 3.13.8 shows a simplified batch history management model.

Advances in relational databases allow data for the process control of current batches to be linked to the histories of previous batches. Using standard query language (SQL) calls to access batch history provides new ways to

FIG. 3.13.7 Production scheduling model.

analyze and report batch histories. Other analysis techniques, such as statistical process control (SPC) and statistical quality control (SQC), can be applied at this level.

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