Williams Biorreactor

Bioreactions

John A. Williams, PhD, P.E. EPS: Environmental & Production Solutions, LLC

T

raditionally, microbiologists have played the dominant role in bioreaction development, with assistance from those in multiple disciplines, including biochemists, geneticists and chemical engineers. And while the fermentation process — the precursor to modern bioreactions — has been used sinceprehistoric days, the major advancements of the last half century have had as much to do with technology as with biology. It is our objective to illustrate the relevance of established chemical engineering practices and processes as they apply to today’s bioreaction engineering, as chemical engineers make further inroads into a field once thought to be the sole domain of biology-based scientists. Thisarticle will discuss the key engineering issues in bioreactor design and operation, focusing on the similarities between traditional chemical reactor engineering and bioreaction engineering. Our objective is not to trivialize the field of biological-based processes; it is simply to demonstrate that strong parallels exist between the two fields. In fact, we will also discuss the key differences thatmust be taken into consideration for successful bioreactions. In so doing, we will provide chemical engineers with increased confidence in applying their knowledge and experience with chemi-

A handful of basic bioreactor designs is used to produce a wide range of products, from antibiotics to foods to fuels. Here’s how to pick the best options for your application.
cal reactors to bear on theissues and opportunities available in bioreactor engineering.

The distant and recent past Long before anyone understood the concept of bioreaction, humans were taking advantage of its results. Bread, cheese, wine and beer were all made possible through what was traditionally known as fermentation — a little-understood process, successful more by chance than design. It was, in fact, the failureand frustration of French vintners who found they were too often producing vinegar not wine, that led the famous French chemist and microbiologist Louis Pasteur to study the fermentation process at their request. What Pasteur discovered was that fermentation occurred as a result of the biological activity of a microscopic plant called yeast. When unwanted microbes infiltrated the wine and “fed”on thealcohol produced by

34

www.cepmagazine.org

March 2002

CEP

the yeast, the microbes left behind distasteful and harmful wastes, which ruined the wine’s flavor. Pasteur’s work laid the foundation for bioreactors as we know them today, because once the process was identified and understood, it could be controlled. And it is the control of the process that concerns chemical engineersfirst and foremost. The scope of bioengineering has grown from simple wine-bottle microbiology to the industrialization of not only beer, wine, cheese and milk production, but also the production of biotechnology’s newer products — antibiotics, enzymes, steroidal hormones, vitamins, sugars and organic acids.

Bioreactors vs. chemical reactors By definition, a bioreactor is a system in which abiological conversion is effected. Although this definition can apply to any conversion involving enzymes, microorganisms, and animal or plant cells, for the purposes of this article, we will limit the definition. The bioreactors referred to here include only mechanical vessels in which (a) organisms are cultivated in a controlled manner and/or (b) materials are converted or transformed via specificreactions. Quite similar to conventional chemical reactors, bioreactors differ in that they are specifically designed to influence metabolic pathways. Traditional chemical reactor models and designs that may be used for bioreaction as well include: continuous stirred-tank reactors, continuous flow stirred-tank reactors, and plug-flow reactors, singularly or in series; ebullized-bed (i.e., “bubbling and...