Wednesday, 19 February 2014

Conceptual Design of Chemical Processes: Douglas

Someone suggested to me that the approach to "Process Design" elaborated in academia can be traced back to a seminal book- this one. As the title suggests, it attempts to design chemical processes, rather than process plants. Its author understands that the expert designer proceeds by intuition and analogy, aided by back of the envelope calculations, but sees the need for a method which helps beginners to cope with all of the extra calculations they have to do whilst they are waiting to become experts.

The arguments underlying the academic approach since built on it are helpfully set out explicitly. It assumes that the purpose of conceptual design is to decide on process chemistry and parameters like reaction yield. Choices between technologies are not considered. Pumps are assumed to be a negligible proportion of the capital and running cost, and heat exchangers are assumed to be a major proportion of capex and opex.

It is implicit in the chain of assumptions used to create the simplified design methodology that a particular sort of process is being designed. Like all design heuristics, it has a limited range of applicability. Though it mentions other industries, it is based throughout upon an example taken from the petrochemical industry, and it is clear that it is most suited to that industry.

Having declined to consider many items which are of great importance in other industries, it finds time for pinch analysis, which was quite new when the book was written. Perhaps this really was a worthwhile exercise for the beginning process designer in the petrochemical industries of the 1980s, but there are many process plant designs in 2014 which do not have a single heat exchanger. There are many industries where process chemistry is given to chemical engineers by chemists, and considering process chemistry is not an engineering task.

This book in itself seems to be a plausible approach to the limited problem it sets out to solve, few of whose assumptions I can argue with in the context of its chosen example. It attempts to offer a beginner a way to choose between potential process chemistries and specify the performance of certain unit operations in a rather old-fashioned area of chemical engineering.

The problem it offers a methodology to solve is however not one I have ever been asked to find a solution for. When I am asked to offer a conceptual design, I am being asked to address different questions, on plants with a different balance of cost of plant components. Petrochemical plants of the sort used as the example in this book don't really get built in the developed world any more. 

The approach in the book does however hang together in a way more recent developments based on it do not. A good amount of effort goes into as rigorous a costing as is possible at the early design stage (ignoring the issue of the items which are left out).

This book in essence seems to contain the slight wrong turns which led by successive oversimplifications and misunderstandings to the utterly unrealistic approaches common in academia nowadays.

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