Process Definition Diagram (PDD)

1. Purpose

The Process Definition Diagram is a tool that enables process technologists to describe a process independently of scale and equipment. It is a form of State Task Network, describing the process as a sequence of tasks that are performed to transform starting materials into products.

In addition to the sequence of tasks, PDDs also capture information on:

Where process materials are introduced and/or removed from the process
The phases present throughout each task
Phase changes (e.g. dissolution, gas evolution, etc)

PDDs can be used to:

Facilitate communiction in multidisciplinary teams as an aid to scale-up or tech transfer
Compare and contrast process options as part of whole process design
Collate observational data for troubleshooting
To represent the baseline process as part of Whole Process Analysis

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2. Information requirements

Information necessary to develop a PDD

Additional information which may be captured as annotations to each PDD task

Process recipe

Process outline - may be conceptual, or based on lab procedures or batch instructions

Phases present in each task

Raw materials, products, intermediates, transient species

Conditions (temperature, pressure)

Energy usage

Waste products and disposal routes

Yields (per task)

Time or cycle time (per task)

Anecdotal information for each phase

Manpower requirements

In-process checks or analysis

Partition coefficients (for separations)

Hold points

Storage/ transport

Unusual phase behaviour and its importance/ impact

Material specifications (e.g. information on crystal form, polymorphism for crystallisations)

Costs - fixed/ variable; capital/ operating

3. Procedure

3.1 Drawing PDD task boxes

1. Draw a box to represent each task as shown.

2. Give each box a unique identifier. It is suggested to number in steps of 10 initially, so that additional tasks can be inserted as the PDD evolves.

3. Give each task a description. Where possible these should be generic descriptions of operations (e.g. Separation) rather than references to specific unit operations (e.g. Distillation).

4. Show materials entering the task from the left hand side of the box (unless a counter-current operation is being depicted). In this example A represents a continuous or all-in feed, whereas B represents an injected or fed-batch feed.

5. Show materials leaving the task from the right hand side of the box (unless a counter-current operation is being depicted).

6. Show energy streams being added to or removed from the task at the top of the box.

7. The width of the box represents a time axis, useful for representing changes in states.

3.2 Showing states on a PDD

1. Use different colours and line styles to represent different material states

2. Overlaid colours and line styles can be used to represent multiphase streams (lesser phases are overlaid)

One of the big advantages of PDDs is that the phases present in each processing task can be represented.

Here task 50 shows an exothermic reaction in a nitrogen-inerted vessel where a solid reactant gradually dissolves as the reaction proceeds, with the final solids dissolving about half-way through the reaction. For operations carried out in stirred tanks, the team can choose whether or not to show the atmosphere at the top of the tank: it makes sense to do this if there might be significant interactions between the liquid and the headspace gas.

Task 80 shows an evaporation, where the liquid product is a concentrated solution.

Task 130 shows an unseeded cooling crystallisation where nucleation occurs at a point some time after cooling commences.

Task 140 shows a filtration under nitrogen pressure. Note that the proportions of phases do not change in a filtration.

3.3 PDD for Solids Processing

For solids processing, a semi-pictorial approach is useful in representing the material states present in commonly encountered unit operations.

3.4 Developing PDD State Task Networks - Coffee Example

PDD boxes are linked together as a series of material states (1) and process tasks (2) to describe the "molecular experience" of the materials in the process.