Fermentation Technologies
Packed-Bed Batch Biofilm Reactor: Concept Analysis

The Biofilm Bioreactor has found extensive uses both in experimental environments for evaluating reaction engineering data as well as in production industrial environments. Often though, the industrial bioreactors have been scale-ups of the experimental bioreactors; and such industrial bioreactors have also almost always been plagued with product quality and production issues. The reality is that, the translation into an industrial reactor of an experimental design developed strictly for the purposes of acquiring engineering data is very tasking at best,  given the divergence of objectives. Moreover, with most of those experimental bioreactors,  the design and operation of the reactors have been semi-continuous or at least not fully batch: There have been removal and addition of reaction feed during the progress of the reaction, notwithstanding the primary definition of a Batch reactor as not accessible for the addition and or removal of reaction-fluid for the duration of the reaction.

However, as elicited by analysis, a biofilm bioreactor is best suite to be operated as a biofilm heterogeneous bioreactor. Obviously, given the current need for urgently developing ethanol fermentation reactors, in view of the global energy crises and concerns of Global warming, an analysis aimed at assessing the viability of using batch heterogeneous  biofilm fermentation reactor should facilitate the decision making processes. Besides, as there is no particular stipulation that the batch reactor can not be heterogeneous, hence the heterogeneous batch biofilm reactor configuration implemented as a packed bed reactor is a viable alternative. Even then, the development of a packed-bed ethanol fermentation reactor for on experimental reactor, from which to evolve a production reactor should begin with a thorough concept analysis of the potential prevailing dynamics.

The Concept-Reactor Configuration
A configuration of a concept Biofilm Batch Packed-Bed Fermentation Reactor adopted for the purposes of such analysis has as the base technology, the design of the batch heterogeneous, subjected to further modifications: The reactor is essentially a cylinder; The body-cylinder is capped at both ends with spherical caps with flanges that are fastened to flanges on the cylinder; At the bottom cap is a pipe connected to a reversible pump that is connected to a feed tank and a product-storage tank; On the top cap is a pressure relief control valve that releases Carbon dioxide at a pre-set pressure; Inside the cylinder at some depth from the top is placed a bed of beads on which have formed biofilm of Zymomonas mobilis, a fermentation bacteria. The bed height is made variable so as to be changed on the basis of the results of the analysis, so while the bed is at a level of the bottom cylinder-flange its height to the top cylinder-flange can and do vary.

Most importantly as a specification of the general design rationale, the design of the bioreactor must have an integrated Fermentation Mash Feeder equipment. The specific configuration of the

Other special features are left out to keep the design generic as well as allow customization for user-specificity of the final design.

Starting the operation
The reactor is opened by opening the top cap - all but one screw-bolt is removed to enable the cap to be pushed to open the reactor. The packed-bed of biofilm beds is inserted into the reactor and settled. The cap is then pushed back into place and the screw-bolted locked and tightened.

Then the  reaction feed of a volume such as will completely cover up the packed bed is pumped with the feed-pump into the reactor from the feed tank. The feed has substrate of glucose, in this special case, and all the substances required to enable the microbes perform the fermentation.

Reactor Dynamics Concept-Analysis
On the start of the analysis the feed has been charged into the reactor and at a rate that is fast enough to overwhelm the initial reaction during the charging of the feed. The reaction mixture is assumed to be quiescent. The mixture is not stirred so as to maintain, for this thought analysis, the stagnation fluid condition preferred to allow spontaneous self-immobilization of the new cell bacteria produce in course of the reaction.

The reaction now starts from within the bed. The substrate diffuses downwards from above the bed into the bed under the chemical potential gradient created by the reaction in the bed. In the counter direction, ethanol produced by the reaction diffuses upward into the fluid above the bed again due to the chemical potential gradient caused by the reaction. Similarly, dissolved carbon dioxide molecules, also created from the reaction, also travels upwards into the bulk of the fluid above the bed. Meanwhile the bacteria grows in a manner described by the Monod equation. The new cells of bacteria also, as expected, self-immobilizes onto the carrier beads and remain in the sessile state but carries on the fermentation as the predecessor microbes. So far the standard expected occurrences of the fermentation reaction obtain.

However, as the new cell of bacteria self-immobilizes, the interstices between the beads of the bed shrinks in size and more fluid is pushed out into the outside of the bed. The biofilm interstices of the microbes also shrinks increasing the rate of fermentation and hence for a while higher production of ethanol, but now feeding on smaller per [microbe] capita of the substrate soon begins to suffer relative reduction of the production of ethanol, and then inhibition takes hold on the microbes.

Meanwhile another possibility looms that may or may not happen. The dissolved carbon dioxide at a certain thermodynamic condition begins to

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 undergo homogeneous nucleation and spontaneously forms bubbles and creates a two phase system, in which the carbon dioxide bubbles are traveling upwards and out of the reaction mixture into the space above the reaction mixture.

The fluid dynamics of the quiescent reaction mixture that prevailed at the start now no longer holds. The upwards traveling bubbles cause limited convective flow of the fluid upwards  and invariably causes recirculation of the fluid from the bottom of the reactor to the top of the fluid, an upwelling of sorts, and eddies in some places. The reactor now suffers spontaneous fluid flow effects, and the transport phenomena of the substrates to the microbe biofilm has changed and must be recaptured in the analysis.

Impact of Concept-Analysis
 Obviously, this spontaneous change of the fluid dynamic characteristics may have to be fully captured, and the model equations of the computational analysis must be such that the fluid must remain quiescent until the right time. Several many characteristics of the reactor can also now be inferred from this one change of behavior and correspondingly a change of the reactor behavior: The height of the packed bed will impact whether the gas bubble formation occurs or not and a reactor designer not desirous of such bubble may shorten the bed height and use multiple reactors, the choice are so vast, there is just no fully addressing all of them. Yet the reactor computational design must now anticipate and incorporate all into the design and test for all and every one of the expected features.

Indeed, the performance of the concept-analysis reveals issues that because a straight up conduct of the equipment design may just miss this dynamics and there would be no obvious reason to plan for and accommodate it in the design given that the occurrence of the dynamics is dependent on certain operating conditions may not have been obvious during the experimental analysis stage of the reactor development, and the computational design modeler must get all these accurately so that a control model can be developed for the control of the reactor.

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