Scheduling (production processes)

Scheduling is an important tool for manufacturing and engineering, where it can have a major impact on the productivity of a process. In manufacturing, the purpose of scheduling is to minimize the production time and costs, by telling a production facility what to make, when, with which staff, and on which equipment. Production scheduling aims to maximize the efficiency of the operation and reduce costs.

Production scheduling tools greatly outperform older manual scheduling methods. These provide the production scheduler with powerful graphical interfaces which can be used to visually optimize real-time workloads in various stages of production, and pattern recognition allows the software to automatically create scheduling opportunities which might not be apparent without this view into the data. For example, an airline might wish to minimize the number of airport gates required for its aircraft, in order to reduce costs, and scheduling software can allow the planners to see how this can be done, by analyzing time tables, aircraft usage, or the flow of passengers.

Companies use backward and forward scheduling to allocate plant and machinery resources, plan human resources, plan production processes and purchase materials.

Forward scheduling is planning the tasks from the date resources become available to determine the shipping date or the due date.

Backward scheduling is planning the tasks from the due date or required-by date to determine the start date and/or any changes in capacity required.

The benefits of production scheduling include:

• Process change-over reduction
• Inventory reduction, leveling
• Reduced scheduling effort
• Increased production efficiency
• Labor load leveling
• Accurate delivery date quotes
• Real time information

Productivity

Productivity is the relation between quantity of inputs and quantity of output.

Inputs

Inputs are plant, labor, materials, tooling, energy and a clean environment.

Outputs

Outputs are the products produced in factories either for other factories or for the end buyer. The extent to which any one product is produced within any one factory is governed by transaction cost.

Within the factory

The output of any one work area within the factory is an input to the next work area in that factory according to the manufacturing process. For example the output of the cutting room is an input to the sewing room.

For the next factory

By way of example, the output of a paper mill is an input to a print factory. The output of a petrochemicals plant is an input to an asphalt plant, a cosmetics factory and a plastics factory.

For the end buyer

Factory output goes to the consumer via a service business such as a retailer or an asphalt paving company.

Resource allocation

Resource allocation is assigning inputs to produce output. The aim is to maximize output with given inputs or to minimize quantity of inputs to produce required output.

Batch Production Scheduling

Background

Batch production scheduling is the practice of planning and scheduling of batch manufacturing processes. See Batch production. Although, scheduling may apply to traditionally continuous processes, such as refining, it is especially important for batch processes such as those for pharmaceutical active ingredients, biotechnology processes and many specialty chemical processes. Batch production scheduling shares some concepts and techniques with finite capacity scheduling which has been applied to many manufacturing problems ^[5]. The specific issues of scheduling batch manufacturing processes have generated considerable industrial and academic interest.

Scheduling in the Batch Processing Environment

A batch process can be described in terms of a recipe which comprises a bill of materials and operating instructions which describe how to make the product. The ISA S88 batch process control standard provides a framework for describing a batch process recipe. The standard provides a procedural hierarchy for a recipe. A recipe may be organized into a series of unit-procedures or major steps. Unit-procedures are organized into operations, and operations may be further organized into phases.

The following text-book recipe illustrates the organization.

• Charge and Mix materials A and B in a heated reactor, heat to 80C and react 4 hours to form C.
• Transfer to blending tank, add solvent D, Blend 1hour. Solid C precipitates.
• Centrifuge for 2 hours to separate C.
• Dry in a tray dryer for 1 hour.

A simplified S88-style procedural organization of the recipe might appear as follows:

• Unit Procedure 1: Reaction
• Operation 1: Charge A & B (0.5 hours)
• Operation 3: Blend / Heat (1 hour)
• Operation 4: Hold at 80C for 4 hours
• Operation 5: Pump solution through cooler to blend tank (0.5 hours)
• Operation 5: Clean (1 hour)
• Unit Procedure 2: Blending Precipitation
• Operation 1: Receive solution from reactor
• Operation 2: Add solvent, D (0.5 hours)
• Operation 3: Blend for 2 hours
• Operation 4: Pump to centrifuge for 2 hours
• Operation 5: Clean up (1 hour)
• Unit Procedure 3: Centrifugation
• Operation 1: Centrifuge solution for 2 hours
• Operation 2: Clean
• Unit Procedure 4: Tote
• Operation 1: Receive material from centrifuge
• Operation 2: Load dryer (15 min)
• Unit Procedure 5: Dry
• Operation 1: Load
• Operation 2: Dry (1 hour)

Note that the organization here is intended to capture the entire process for scheduling. A recipe for process-control purposes may have a more narrow scope.

Most of the constraints and restrictions described by Pinedo^[9] are applicable in batch processing. The various operations in a recipe are subject to timing or precedence constraints that describe when they start and or end with respect to each other. Furthermore, because materials may be perishable or unstable, waiting between successive operations may be limited or impossible. Operation durations may be fixed or they may depend on the durations of other operations.

In addition to process equipment, batch process activities may require labor, materials, utilities and extra equipment.