Bioreactors vs. chemical reactors
By deﬁnition, a bioreactor is a system in which a biological 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 speciﬁc reactions.
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 boiling”) reactors; and ﬂuidized-bed reactors. The term “bioreactor” is often used synonymously with “fermenter;” however, in the strictest deﬁnition, a fermenter is a system that provides an anaerobic process for producing alcohol from sugar.
Bioreactors differ from conventional chemical reactors in that they support and control biological entities. As such, bioreactor systems must be designed to provide a higher degree of control over process upsets and contaminations, since the organisms are more sensitive and less stable than chemicals. Biological organisms, by their nature, will mutate, which may alter the biochemistry of the bioreaction or the physical properties of the organism. Analogous to heterogeneous catalysis, deactivation or mortality occur and
promoters or coenzymes inﬂuence the kinetics of the bioreaction. Although the majority of fundamental bioreactor engineering and design issues are similar, maintaining the desired biological activity and eliminating or minimizing undesired activities often presents a greater challenge than traditional chemical reactors typically require.
Organisms, inﬂuenced by their morphology and the bioreaction medium, are shear-sensitive to varying degrees.
A number of bacteria, yeast and fungi cultures that can be relatively tolerant of high-shear environments exhibit a robustness in high-energy mixing vessels. Animal, ﬁsh, insect and plant cells are delicate and usually require low-shear environments for viability. The viscosities of bioreaction masses may change during growth and production phases, and, often, the medium becomes non-Newtonian as a cycle progressed. Mixing within the bioreactor is integral to efficient heat and mass transfer during the production phases, which places additional constraints on the suitable agitation mechanism and rheology of the bioreaction medium.
Other key differences between chemical reactors and bioreactors are selectivity and rate. In bioreactors, higher selectivity — that is, the measure of the system’s capability for producing the preferred product (over other outcomes) is of primary importance. In fact, selectivity is especially important in the production of relatively complex molecules such as antibiotics, steroids, vitamins, proteins and certain sugars and organic acids. Frequently, the activity and desired selectivity occur in a substantially smaller range of conditions than are present in conventional chemical reactors. Further, deactivation of the biomass often poses more severe consequences than a chemical upset.
Rate is of secondary importance. For many biological systems, an incubation period is needed to prepare a culture used to inoculate the bioreactor with the producing microbes or their precursors. Although a bioreaction can be brief, in systems where organism or biomass growth is necessary, the bioreaction can take 10–20 d for completion of the batch. Further, the bioreactor should not be regarded as an isolated unit, but as part of an integrated unit operation with both upstream (preparation) and downstream (recovery) unit operations.