Back when I was writing my book on Plant Layout, I surveyed other practising engineers on the issue of staging of plant design, asking how many stages they thought there were in the design of a plant.
My survey respondents generally thought there were somewhere between three and ten, depending to some extent on what they thought I meant by a ‘stage’.
However, it was quite noticeable that many engineers believed their view on the matter was the only correct one, whilst non-practitioners were equally certain that their view was also correct.
Most engineering practitioners have to work with a stage-gate process, which is set by the management of the organisation ultimately responsible for paying for/operating the plant.
Precisely which stage gates are chosen is driven by a combination of the regulatory environment and prevailing culture within the company, industry or country of the plant in question.
If we are working in a research/science led industry such as pharma, there will be more up-front ‘sciencey’ stages, followed by a mad rush to go from lab scale to “full scale” without much thought of optimisation.
The full scale plant will probably just be the lab process only with bigger vessels. Nowadays the kit may even comprise a collection of single-use disposables.
The rush to make as much money as possible before the patent runs out militates against spending the time necessary to design a reasonably optimal continuous process.
The high profit margins in some of these sectors mean that this approach makes a kind of sense, though much less so in food and drink, where the products are more low margin commodities.
There are moves to address this anomaly, but for now, this is how staging looks in the food and drugs industries.
In the traditional chemical process industries (CPI), centred on bulk organic chemical manufacture and oil and gas, lab scale science and research play less of a part.
Instead, software process modelling fills additional upfront stages, especially in the retrofits, plant extensions etc. which form the bulk of the work in this sector in the West nowadays.
This approach tends to carry with it the US cultural terminology, and units which were exported worldwide through the oil and gas industry. I tend to find that it is those who work in the CPI who are most convinced that their ways of working, terminology etc. are correct, as they are the ones apparently doing ‘proper’ chemical engineering.
Then there are the environment industries (in which I tend to do most of my work) where much of what chemical engineers do nowadays was done historically by civil engineers.
We have inherited some of their culture although, as staging of design is a feature of all engineering activity, what we do does not differ significantly from the previous two approaches.
However, we tend to have neither labwork nor modelling to support our design activity. We must consequently rely more on heuristics, and are under a lot of resource pressure as we work in a low margin environment. We therefore tend to have fewer stages of design, and a shorter design programme.
One thing I found from my survey was that everyone who practices engineering knows that there are three basic stages of design prior to construction.
We can call these conceptual, front end engineering design (FEED), and detailed design. We might split conceptual design into a number of sub-stages.
We might differentiate between detailed design and design for construction. We might include redesign during and after construction, and so on. But whether we are designing a process plant or a wedding cake, this is how it goes. Apparent differences mask an underlying similarity. This has always been true, and always will be true.
But what about those who have never practised? What do academic “chemical engineers” think? Some apparently believe that the differences mean that there is no underling similarity, that there is no necessary progression, that things can be rearranged anyway that suits.
Academics often teach an approach to process design which entirely ignores the commercial reality of engineering. Some admit that they are teaching how it ‘should be’ rather than how it is, but many do not, and instead try to make a virtue of ignoring how things are, claiming that they are teaching how things will be in future.
This is of course nonsense. When I wrote my plant layout book, updating it from a first edition published in the 1980s, I found that nothing significant had changed in how design was done since then.
We have new tools, but we have the same design problems to solve, and there is a natural order in which to do so. Process design is no more maths and science than it ever was.
For readers who graduated fairly recently, consider how ludicrous “Process Synthesis and Design” is in the context of engineering practice. Has anyone ever given you a clean sheet and suggested that you singlehandedly design a full-scale plant from scratch?
In the unlikely event that they did, did they expect to you to devise your own process chemistry, use multiple novel processes never used at the proposed scale before, model the whole thing in Aspen Hysys without supporting process trials, carry out a pinch analysis on the result and optimise for maximum product yield and minimum input energy without consideration of cost, safety, plant footprint, or the requirements of other engineering disciplines? No? So what was the point of those “process design” modules you did at university?
What does this have to do with staging? The exercise undertaken by chem eng students in design projects is at best a FEED study, but is more often no better than conceptual design, to the extent that it bears any resemblance at all to professional design. Projects are staged to avoid wasting money on engineering an unviable design.
The “Process Synthesis and Design” approach they are being taught is virtually all wasted effort. The main thrust of conceptual and FEED stages is a viability check. Detailed design costs a lot of money. If the process proves, upon investigation by design, to be insufficiently cost effective, robust and safe, then we abandon the project.
This is in fact the most likely outcome. Applying just enough effort to establish these things to the required degree of certainly is what we are employed for, and this is where design staging plays its part. Teaching an approach which ignores staging is a waste of the students’ time.
