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More Update
Post: 09_05_2008; 01_29_2009; 03_29_2009
Given the intense interest in the
production of biodiesel both by hobbyists and commercial
producers, there are large quantities of glycerol, the
by-product of the transesterification reaction, that is also
produced. Depending on the type of alcohol used:
Methanol or
Ethanol; the
glycerol is not readily usable in making soap - the primary use for
glycerol. Irrespective of the alcohol that is used, some traces of
the alcohol will always be left in the glycerol, hence given the
toxicity of methanol, the glycerol produced with methanol may not be
used for soap. Yet, the product must be used in some other form that
support the overall objective of biofuel production - the reason for
the production of bio-diesel to begin with. The conversion of the
glycerol into biofuel presents one such option of the utilization of
the by-product.
The bioprocess design and
assembly of process equipment for the fermentative utilization of
glycerol for ethanol fermentation is based on the
fermentative utilization by E Coli of glycerol, and prevailing
knowledge of production bio-diesel defining the content of the
glycerol feed which depending the method of
quenching of the biodiesel reaction may essentially be a mixture of
glycerol, reaction residual alcohol either of methanol or ethanol, water and
possibly some sodium (or Potassium) hydroxide. All ethanol fermentation
processes, however, may be composed of
two components: Fermentation Feed Process - a feed-specific feed process
component, and the Fermentation Process proper, and glycerol
as a feed in this case presents very peculiar processing needs stemming from the method of
production. As such the process engineering, as should be with every
such consideration, must start
with a thorough analysis of the glycerol production process and
consequentially feed-specific processing.
Glycerol Feed Process
As noted previously the
contents of a glycerol feed is determined by the Biodiesel
Production process - as impacted by the method of reaction quenching
- from which the feed obtains. Limited
quenching methods analyses have shown that depending on the biodiesel production and
reaction quenching approach, there may or may not be hydroxide
to consider in the process design; and there are however, several methods for
quenching the reaction that would likely remove the need to consider
hydroxide. Yet worst case scenario analysis suggests the analysis of
the processing of glycerol feed that contains water, alcohol -
either methanol or ethanol and hydroxide. The rationale for adopting
the worst case scenario for analysis is supported by the reality
that often, a production operation may have to buy the feed from
several sources all of which may not be supplying the same quality
raw material.
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The glycerol feed process
component as such has the object of separating hydroxide, alcohol
and water from the feed. An obvious process equipment of the process
is, in this case, an Ion Exchange equipment, through which the
glycerol feed is pumped to remove the hydroxide, the preferred
transesterification reaction catalysts. The feed must also be
treated a feed into a distillation process, by which the alcohol is
distilled off. The particular sequence is not critical, however. The
distillation process may, however, be designed to also remove the
any water in the glycerol, which is known to be hygroscopic, and as
such is quite likely to contain water.Glycerol Fermentative Utilization
The conceptualization of a
process based on the considerations elicited so far suggests the following set of
chemical process equipment set:
Glycerol Storage Tank
Ion Exchange Vessel
Glycerol [Purification] Distillation
Ethanol Distillation
Fermentation Reactor
Ethanol Storage Tank
Operationally, the process very simply entails
pumping the glycerol in the Storage tank into the Ion Exchange
Vessel, and then into the Glycerol Purification Distillation
Still to further remove any remnants of alcohol-residue of the
biodiesel process as well as water. Subsequently, the glycerol which
is collected as the bottom of the Distillation Unit is pumped into
the Fermentation Reactor. The effluent reaction mixture obviously
now containing high concentration of ethanol, as a result of the
fermentation, is then pumped into the Ethanol distillation
Unit, albeit a Flash Distillation Column. In the distillation
process, the ethanol is collected as the distillate and any
non-utilized glycerol is collected as the bottom, remixed with the
feed to the fermentation reactor and effectively recycled; while the
ethanol collected as the distillate is pumped into the Ethanol
Storage Tank.
Beside the issue of the
contents of the glycerol feed and the attendant need for specialized
processing, the nature of the fermentation microbe is of concern.
The fermentation microbes, E Coli, for deployment in the process
is a known pathogen, which imposes that preferably homogeneous
bioreactor not be adopted for this process. So accordingly, the
bioreactor as conceptualized is a heterogeneous reactor, with the
immobilized
microbes occluded within carrier beads. More specifically, the
bioreactor is by design a
Moving Bed Bioreactor
- with the general characteristic of the reaction-fluid moving
upwards, while the solids, in this case the microbes immobilization
beads, are falling down through the broth.
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Of course, the choice of the Moving Bed Bioreactor is an engineering
judgment call; however, this form of bioreactor is perhaps the most
appropriate under the circumstances. Admittedly, superficially, any other type of bioreactor:
Batch,
single and
multi-
Packed bed bioreactors, or
heterogeneous bioreactor; may appear to be equally operational,
however, this is really not so.
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First, the need to control the E
Coli microbes growth while inside the reactor suggested that the
carrier beads be removed before the microbes grow out of the
interior of the carrier-beads. After all,
immobilized microbes dynamic analysis shows that such growth is
the norm and in fact occurs.
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Second, the Moving Bed Reactor, further supports the continuous
operation of the bioprocess, by enabling the automating of the
microbes immobilization beads re-conditioning for
reintroduction into the bioreactor, and thereby eliminates the
need for repeated contact with the E Coli carriers as would be
necessary with the use of Batch reactor which need recharging
after the reaction time of each batch production-run.
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Third and finally, this bioreactor provides maximum conversion of
the glycerol into ethanol because the reaction mixture or broth is
continuously being exposed to fresh microbes that are at their
most fermentative state.
However, as shown the
adoption of a specially designed process streams premixer,
Mash Feeder, is recommended, in order to eliminate the potential
impact on reactor performance resulting from reaction mixture
heterogeneity for both homogeneous and heterogeneous bioreactors.
The configuration of the Mash Feeder must be based on the general
requirements of nutrients
for a
fermentative glucose utilizing reactor. By the nutrients
requirements, the configuration of the Mash
Feeder
for the utilization of glycerol therefore will have about, say eighteen feed-ports, pumping liquids into the substrate. Further, the Mash Feeder
configuration must be such as to dissolve the proper concentration of the
oxygen into
the mash just before discharge into the reactor; given that
even the oxygen concentration in the mash must not be allowed to
fall below the optimal concentration at exit of the broth into the
bioreactor.
Clearly with appropriate
care and reasoned consideration of the production and safety factors
involved, a large scale fermentative glycerol utilizing
ethanol production process can be developed.
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