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Plant Design of Unit Operations

By: , Posted on: July 27, 2015

Many books on process plant design overemphasize the detailed design of unit operations which is beloved of academics, but is not much to do with professional process plant design. Mine does devote a chapter to it:

Design Of Unit Operations

We do not wish to spend any more time on design at each stage than is necessary to progress the overall design. Rule of thumb design is therefore the norm for process plant designers.

We do, however, use more onerous but accurate heuristics as it grows increasingly likely that someone will actually build the plant.

If you are working as a process designer, there will mostly (but not always) be a design manual which will give the relevant rules of thumb for designing the items you are being asked to design which encapsulates the company’s experience in the area.

Failing that, there will be a more experienced engineer in your department or at least in your company who knows the rules of thumb. If there is neither a manual nor such a person, this is a bad sign. You aren’t going to learn much in this company.

If there is such a person, they may not be willing to share their knowledge. This isn’t a great sign, but sooner or later the company will have to give you support if they want you to do a competent job.

The more experienced engineer is often the world’s leading expert on the particular job you have to do, as they don’t just know a way to design the plant, but they know how to design the plant so that your company can build it. So be nice to him or her, as they can teach you more than University ever did.

But what if you don’t have a manual or a Yoda? As well as Perry’s Handbook, there are books which contain general rules of thumb- I give a couple of good ones in the reading list at the end of the chapter. These are not going to be as good as the experienced engineer, but they may well be more useful than attempting to use some of the theoretical approaches they taught you at University.

There are some recent books offering “rules of thumb” generated by modeling and simulation programs. Don’t use these. Proper rules of thumb come from experience with multiple full scale real world plants, not first-principles computer programs. First principles design doesn’t work.

Approaches to Design of Unit Operations

First Principles Design

Don’t ever do this in normal professional process plant design practice. Even if you had enough data to allow you to design a unit operation from first principles, it would be at best a prototype, and your employer would be the one offering the process guarantee by making it part of the plant they were guaranteeing.

Design by Simulation Program

Despite all that I have said about the use of simulation at whole-plant level, unit operations are less complex than whole plants, and some suppliers now offer you simulation program blocks which they have verified and tuned to match their real equipment (though not validated for your application).

In this way the most normal of normal process design activity can become rather similar to building a Lego model. I am not, however, sure that we can call this activity process plant design. It seems to me more like equipment design, where a whole plant may be specified as a collection of standard equipment.

Design from Manufacturers’ Literature

Since detailed design involves putting together unit operations you can actually buy, manufacturers’ catalogues are a useful tool for selecting the unit operations which we put into our plant designs.

Updated catalogues also frequently include new items of equipment we might not have considered if we had not read the catalogue.

Back when I only designed plants for a living, reading through the pile of supplier catalogues which had accumulated whilst I was engrossed in designing and pricing the last plant was a very useful way to spend the time waiting to be allocated my next job.

Nowadays these catalogues are more likely to be found on websites rather than in hardcopy, and this is very handy in an academic setting, allowing us to bring realism to our students’ designs without bothering manufacturers with enquires from students who are not actually going to purchase kit.

One thing which we find hard to recreate in the academic setting are interactions with technical sales staff. Their detailed knowledge of their products and their capabilities and limitations can allow plant designers to see new ways to design ourselves ahead.

Civil engineers are, by the way, convinced that picking “gubbins” from catalogues is all process engineers do for a living. This is a point they may make to you when you tell them they can’t have the data they need to proceed with their design for a couple of weeks. (The standard riposte to this is to tell them that whatever you say, they will in any case specify the concrete wall they are going to design to be 150mm of concrete containing two sets of #10M rebar).”

In summary, even where we do get involved in design of unit operations, we don’t do it the way the teach it in university.

Read more from Sean on SciTech Connect


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About the Author

sean moranProfessor 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.

Connect with Sean on LinkedIn here, check out his Facebook page here and stay up-to-date on his thoughts, research and practice at his personal blog here.

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