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Batch Fermentation Reactor, the vessel within which fermentation
reactions are conducted, as generally
analyzed for design rationale, could be designed either as a
homogeneous reactor or as a heterogeneous reactor. The differences between these two types or modes of
operation of fermentation reactors is in the manner of dispersion of
the microbes in the reaction-mixture, usually called Broth or Mash: The reactor
design or mode of operation is deemed homogeneous if the fermentation microbes are simply mixed into the
reaction mixture, and The reactor design or mode of operation is
deemed heterogeneous in the cases for which the
microbes are immobilized on an
effective
carrier and immersed within the broth.
Taking off from the
design rationale analysis,
this report actually analyzes the issues inherent in the design of
homogeneous batch bioreactor - the design option in which the
reaction mixture, or broth or mash is intimately mixed with the
microbes supporting the fermentation. This analysis based on a
concept
design of the generic batch fermentation reactor is an attempt at
critical examination of the design of homogeneous batch fermentation
reactor in the context of the issues that have been suggested for
consideration in such designs.
Homogeneous Reactor Design
Basics
The equipment design of the reactor design task, actually, is the
determination of both the external and internal structure of
the equipment as well as the dimensions, and which for all intents
and purpose, is a manifestation of concept design that was subsumed
in the material balances calculations . So obviously the feed volume
and mass will be the same as was assessed for the material balances. The determination of the
production volumes and substances properties is conducted to ensure
compliance with the Conservation of Mass. Though explicit
calculations are not proffered here, the validation of the methods
presented in another analysis is evaluated.
The
batch reactor
material balance equation of critical consideration, has
been given as
dX/dS = (Km/k)/(K + S) (1)
and is to be integrated from S
= So to (1- ξ)So, where ξ is the final conversion of substrate
required to support operations viability, to get the final
concentration of the microbes; and then obtain the values of the
other reactants - at the end of the reaction - using the applicable
biochemical pathway-based
techniques of
materials evaluation, given that the feed state must also be consistent with the
constituents of
typical ethanol fermentation reactor stream used in the material
balance calculations.
In this context, the equipment design becomes the development of
features for handling different aspects of the operational
consideration of the
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fermentation reactor. As foundation of the reactor design,
then based on the batch reference template, the reactor template has
the form of a Cylinder, In any case, a very simple sizing of the
equipment by rule of thumb proffers the height of the reactor should
be no more than 12-feet, and the interior diameter of the reactor
should be no more than 6-feet, leading to the volume
calculation:
Volume of reactor (Vrxtor)
= π(D/2)2H
where D = 6 and H = 12 as per
the rule of thumb, and then the dimensions are adjusted as per the
reactor design
rationale.
Additional essential
equipment includes stirrer, Mash Feeder, integrated into the Feed
Port as per the special equipment design proffered and attendant of
the reaction-specific needs: Currently, the
mash-related cause of heterogeneity in a homogeneous bioreactor,
stems from the adopted method of separately feeding the mash
constituent substrates and the reactants for the anabolic reactions
and then stirring the mixture:
Operationally, the
preferred reactor state with respect to
optimal reaction engineering demands, however, is one in which the
anabolic reactions only supports cell functionalities without
cell-mass growth. This specification imposes certain demands on the
configuration of the reactor. Particularly noteworthy in this
respect, is that as currently designed these bioreactors have the oxygen required for the
anabolic reactions being dissolved into the mash as per
Henry's Law relative to the concentration above the liquid top
surface ; and therefore the stirring of the mash invariably
exposes the mash-fluid particles to different concentrations of
oxygen depending on whether the fluid particle moves to the surface
or continues to remain below and far below the surface.
Clearly then, uniform
oxygen concentration in the reaction mash during the duration of the
reaction requires that the reactor must be such as to enable the
introduction of air of the desired concentration uniformly in the
reaction mash. In effect, then the efficacious reactor configuration
must also
introduce Oxygen of the desired concentration, such that the mash
which is even
far below the surface has the same concentration of oxygen, for
supporting uniformity of reaction intensity everywhere in the mash.
This class
of bioreactors also exhibit mixedness heterogeneity: Mixing of the
microbes with the broth almost certainly never achieves a state
of well-mixedness and as such the reaction mixture does not have
uniform concentration through out the reactor: Research on mixing
shows that generally, the mixing of liquids poured into a vat
together before mixing, attains the state of well-mixedness only
over an exponential time span, because, the heterogeneity of the
state of mixedness decays over that span; and as a result the
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operations of this reactor
must plan for and account for heterogeneity. A viable remedy to this
situation is the introduction of a Pre-reaction Mixing Stage,
coupled with the reduction of the reactor temperature while
stirring. So required to be implemented is a Pre-reaction
Ignition Stirring Stage aimed at evolving the reaction broth as
close to uniformly mixed condition as possible before the igniting
of the reaction, through raising the temperature.
Computational Design
Analysis of Batch Fermentation reactors are usually
computation intensive and do not submit readily to comprehensive
documentation; and where the Pre-reaction Mixing Stage is not
adopted in the operation, then the Mathematical analysis of this
type of reactor should aim to include various forms of reaction
mixture partitioning coupled with stochastic dispersion in aiming to
capture the heterogeneity.
Further, given that one of the driving
forces for adoption of
heterogeneous reactors as the ease of use of pathogenic bacteria,
the use of such bacterium in a homogeneous bioreactor however, must for the purposes of
developing computational design allow for the operation of the reactor
to continue until the full inhibition of microbes through
asphyxiation.
Designing for the above
delineated features clearly constitutes designing for any
fermentation bioreactor. however, given that the batch reactor
must remain inaccessible throughout the course of the reaction, the
cell mass of the microbes must yet be carefully evaluated prior to
introduction into the reactor, and then after the reaction period. Such evaluation
can again be made by following the method of
calculating the
growth cell-count using the Monod equation. The object of
factoring this possibility is to ascertain the the conditions under
which there is no net cell growth during the reaction actually
obtains..
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