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The Essence of Process Plant Design
I have written five blog posts in which I criticize how design is taught (and isn’t actually done). The remainder of the book deals with how design is done, and therefore in my opinion should be taught. As I say in Chapter 6:
“The very essence of process plant design, the thing people employ chemical engineers to do, is system level design. By this I mean more or less the same thing as Pugh means by his term “Total Design”, rather than the approaches called by similar sounding names in academia.
It is integrative, though I do not mean by this academic “Process Integration”, I mean integration of the needs of all engineering design disciplines, and those who are to build and operate the plant.
It is multidisciplinary, involving usually as a minimum civil and electrical engineers, as well as to a lesser degree construction stage staff, management, clients and plant operators.
It is multidimensional, taking into consideration as an absolute minimum the cost, safety and robustness implications of every decision.
It is iterative- a design evolves through successively better incarnations.
Lastly, the thing which makes it truly system level design is that process plant designers see in their minds eye a complex system working as a whole. This is why all partial approaches entirely miss the point- it isn’t about optimizing any one variable. It is about being able to imagine a completely integrated system which none can fully understand, but that the designer understands well enough to make it do what they want it to do in the way they say it is going to do it.
The main tools for system level design are the P&ID, PFD and GA, which represent a great deal of our design deliberation in a concentrated form. We can see from them at a glance most of the things we need to consider in making the components of our plant work together.
We will use them in slightly different ways as design progresses, but the PFD encapsulates the integration of mass and energy balance, the P&ID system level control and integration, and the GA the physical and hydraulic constraints.
They are not just records of the designer’s thinking, though this is an important function to allow the review of designs by others. Producing these documents forces the designer to consider the issues described in the last paragraph, and allows him to visualize the effects of his proposed solutions. They are therefore design tools as well as records.
At the conceptual design stage, we have at least to get a broad idea of recycle ratios, as these can have a huge effect on main plant item sizing for certain types of unit operations, such as reactors and their often closely associated separation processes. This will involve generating a PFD and associated mass balance.
We need to get an idea of how physically large the plant is going to be, so that we can see if it going to fit on the available site. We will need to produce a GA to do this, and carry out rough hydraulic calculations.
These hydraulic calcs will tell us whether we are going to have a completely pumped system, or make some use of gravity, a choice which will affect the plant layout and be seen on the GA.
The P&ID will be affected by these choices, and there are also choices to be made between software and hardware solutions to design problems, the solutions to which will appear even on early stage P&IDs.
At this stage we are probably designing unit operations using rules of thumb, without checking whether the units we are specifying are commercially available. We probably have fairly sketchy design data, have made quite a few simplifying assumptions, and have been given only a few resources to get to the desired endpoint.
Our aim is to see if we can fit the plant on our site, whether it is affordable and whether it is plausible from the point of view of cost, safety and robustness. This is an initial rough screening, which the overwhelming majority of proposals fail. If we are asked to produce a detailed design which we are willing to stand by as a fairly robust investigation, evaluation and solution of the vast majority of design problems and tasks, we need to look much harder. The same three drawings are, however, still going to be at the center of the exercise, but now they are primarily tools for collaboration with others.
System level design optimizes the whole plant design, considering the implications of design decisions for our civil and electrical partners, installation and commissioning engineers, and plant operation and maintenance staff. We can send drawings (especially the GA which almost everyone can understand) back and forth with our design collaborators, installation contractors and so on to take on board their opinions.”
An Applied Guide to Process and Plant Design is available for pre-order on the Elsevier Store.
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About the Author
Professor Moran is a Chartered Chemical Engineer with over twenty years’ experience in process design, commissioning and troubleshooting. He started his career with international process engineering contractors and worked worldwide on water treatment projects before setting up his own consultancy in 1996, specializing in process and hydraulic design, commissioning and troubleshooting of industrial effluent and water treatment plants.
In his role as Associate Professor at the University of Nottingham, he co-ordinates the design teaching program for chemical engineering students. Professor Moran’s university work focuses on increasing industrial relevance in teaching, with a particular emphasis on process design, safety and employability.
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