It is very helpful to have a good description of how a process works.
Process Descriptions are ideally written with the terminology provided by S88 but this is not essential.
The Descriptions are normally provided for each of the process units in the plant, and for utilities etc
How far these go into detail is another issue however, for example an extremely long and detailed list of steps and valve movements etc all in a text can actually be counter productive. That detail is of course needed eventually, but it is best done using a Requirements Model
Process Descriptions should identify the basic inputs and outputs of the process, what it does (and how) and the critical parameters
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For example a description of a Reaction unit might be like this:
Process Description for Reaction Unit:
This Unit is a mixing vessel with temperature control
Feed material 1 is a liquid
Feed material 2 is a powder containing catalysts
Batches of Stuff
This stuff may be one of several types depending on the recipe used.
The Production Unit can run several recipes and these are not all known. The chemist should be able to design any recipe that is built from the basic steps in any order
Addition of Feed 1
Addition of Feed 2
Temperature control is critical and some processes are exothermic
An example of a simple recipe is
1 Add some Feed 1 to the Production Unit
2 Add some Feed 2
3 Start agitation at Speed1 and raise the temperature to Temp1*
4 Over a period of Time1, Increase the agitation speed to Speed2* and the temperature to Temp2*.
5 Over a period of Time1, reduce the agitation speed to Speed3* and the temperature to Temp3*
6 Transfer the completed batch to the product vessel
During steps 3 to 4 if the temperature cannot be ramped fast enough then the time should be extended using a calculation (to be provided, ramp dwell to be used first)
At the end of this the material is ready and can wait.
Transfer the completed batch
More complex recipes are also possible, for example with a second stage of addition, or even more, or with more complex temperature profiles.
*Note - it is always helpful to have a table of the critical parameters
Often a plant has multiple similar units. Generally you would only need to write one Process Descriptions for each type.
See also Emerson's Resources on what a Process Description should be. Follow the white paper link to "Using a process description to set automation requirements for a Life Sciences project".
Excellent as it is, it is most likely far more detailed than most process engineers might want to write.
Furthermore the high level of detail that they suggest lends itself much more to a Requirements Model